optimizer: move inlineable constants into statement position during compact!-ion

In code like below
```julia
Base.@assume_effects :nothrow function erase_before_inlining(x, y)
    z = sin(y)
    if x
        return "julia"
    end
    return z
end

let y::Float64
    length(erase_before_inlining(true, y))
end
```
the constant prop' can figure out the constant return type of
`erase_before_inlining(true, y)` while it is profitable not to inline
expand it since otherwise we left some `!:nothrow` callees there
(xref: #47305).

In order to workaround this problem, this commit makes `compact!`move
inlineable constants into statement positions so that the such
"inlineable, but safe as a whole" calls to be erased during compaction.
This should give us general compile-time performance improvement too
as we no longer need to expand the IR for those calls.

Author: aviatesk

base/compiler/ssair/ir.jl

@@ -1341,7 +1341,12 @@ function process_node!(compact::IncrementalCompact, result_idx::Int, inst::Instr
                 return result_idx
             end
         end
-        ssa_rename[idx] = SSAValue(result_idx)
+        typ = inst[:type]
+        if isa(typ, Const) && is_inlineable_constant(typ.val)
+            ssa_rename[idx] = quoted(typ.val)
+        else
+            ssa_rename[idx] = SSAValue(result_idx)
+        end
         result[result_idx][:inst] = stmt
         result_idx += 1
     elseif isa(stmt, PiNode)

test/compiler/inline.jl

@@ -2065,3 +2065,16 @@ end
 # https://github.com/JuliaLang/julia/issues/50612
 f50612(x) = UInt32(x)
 @test all(!isinvoke(:UInt32),get_code(f50612,Tuple{Char}))
+
+# move inlineable constant values into statement position during `compact!`-ion
+# so that we don't inline DCE-eligibile calls
+Base.@assume_effects :nothrow function erase_before_inlining(x, y)
+    z = sin(y)
+    if x
+        return "julia"
+    end
+    return z
+end
+@test fully_eliminated((Float64,); retval=5) do y
+    length(erase_before_inlining(true, y))
+end