Fix bug in LowerCallMemmove (#123907)

Might fix (but not sure) #123748

`LowerCallMemmove` was initially written to inline `SpanHelpers.Memmove`
into something that guarantees proper non-aliased move (load all data
into separate registers first, then save them all at once) as opposite
to what normal `GT_STORE_BLK` does - it's paired with
`genCodeForMemmove`. That's why we sort of try to Lower BLK/STORE_BLK by
hands there.

Later, `CORINFO_HELP_MEMCPY` was added as well (typical scenario:
`cpblk` IL code (`Unsafe.CopyBlocksUnaligned`) with the length that
becomes a constant in a late phase) and since it doesn't require the
memmove semantics, we used `GenTreeBlk::BlkOpKindUnroll`, but in that
case we either have to replace the call into a pre-Lower shape and call
Lower on srcBlk+storeBlk since we're no longer paired with
`genCodeForMemmove`, or just use the same memmove mode.
`memmove` mode is more expensive for LSRA and doesn't support addressing
modes, but it seems that `CORINFO_HELP_MEMCPY` is rarely used.

Example of a changed codegen:
```cs
static void Test(ref byte a, ref byte b, uint len)
{
    if (len == 200)
    {
        Unsafe.CopyBlockUnaligned(ref a, ref b, len);
    }
}
```
Was:
```asm
       vmovdqu32 zmm0, zmmword ptr [rdx]
       vmovdqu32 zmmword ptr [rcx], zmm0
       vmovdqu32 zmm0, zmmword ptr [rdx+0x40]
       vmovdqu32 zmmword ptr [rcx+0x40], zmm0
       vmovdqu32 zmm0, zmmword ptr [rdx+0x80]
       vmovdqu32 zmmword ptr [rcx+0x80], zmm0
       mov      rax, qword ptr [rdx+0xC0]
       mov      qword ptr [rcx+0xC0], rax
```
Now:
```asm
       vmovdqu32 zmm0, zmmword ptr [rdx]
       vmovdqu32 zmm1, zmmword ptr [rdx+0x40]
       vmovdqu32 zmm2, zmmword ptr [rdx+0x80]
       vmovdqu  xmm3, xmmword ptr [rdx+0xB8]
       vmovdqu32 zmmword ptr [rcx], zmm0
       vmovdqu32 zmmword ptr [rcx+0x40], zmm1
       vmovdqu32 zmmword ptr [rcx+0x80], zmm2
       vmovdqu  xmmword ptr [rcx+0xB8], xmm3
```


No
[diffs](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1287687&view=ms.vss-build-web.run-extensions-tab)

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: EgorBo

src/coreclr/jit/lower.cpp

@@ -2468,6 +2468,8 @@ bool Lowering::LowerCallMemmove(GenTreeCall* call, GenTree** next)
 
             GenTree* dstAddr = call->gtArgs.GetUserArgByIndex(0)->GetNode();
             GenTree* srcAddr = call->gtArgs.GetUserArgByIndex(1)->GetNode();
+            assert(!dstAddr->isContained());
+            assert(!srcAddr->isContained());
 
             // TODO-CQ: Try to create an addressing mode
             GenTreeIndir* srcBlk = m_compiler->gtNewIndir(TYP_STRUCT, srcAddr);
@@ -2477,11 +2479,8 @@ bool Lowering::LowerCallMemmove(GenTreeCall* call, GenTree** next)
                 GenTreeBlk(GT_STORE_BLK, TYP_STRUCT, dstAddr, srcBlk, m_compiler->typGetBlkLayout((unsigned)cnsSize));
             storeBlk->gtFlags |= (GTF_IND_UNALIGNED | GTF_ASG | GTF_EXCEPT | GTF_GLOB_REF);
 
-            // TODO-CQ: Use GenTreeBlk::BlkOpKindUnroll here if srcAddr and dstAddr don't overlap, thus, we can
-            // unroll this memmove as memcpy - it doesn't require lots of temp registers
-            storeBlk->gtBlkOpKind = call->IsHelperCall(m_compiler, CORINFO_HELP_MEMCPY)
-                                        ? GenTreeBlk::BlkOpKindUnroll
-                                        : GenTreeBlk::BlkOpKindUnrollMemmove;
+            // For simplicity, we use BlkOpKindUnrollMemmove even for CORINFO_HELP_MEMCPY.
+            storeBlk->gtBlkOpKind = GenTreeBlk::BlkOpKindUnrollMemmove;
 
             BlockRange().InsertBefore(call, srcBlk);
             BlockRange().InsertBefore(call, storeBlk);
@@ -2499,7 +2498,8 @@ bool Lowering::LowerCallMemmove(GenTreeCall* call, GenTree** next)
 
             JITDUMP("\nNew tree:\n")
             DISPTREE(storeBlk);
-            // TODO: This skips lowering srcBlk and storeBlk.
+            // We've just lowered srcBlk and storeBlk here and it's now what genCodeForMemmove expects.
+            // So the next node to lower is whatever we have after the storeBlk.
             *next = storeBlk->gtNext;
             return true;
         }