tests: simplify dont-shufflevector-bswaps test

First, give it an appropriate name instead of "issue 122805":
it is about not generating shufflevector when a byteswap would do,
so we call it dont-shufflevector-bswaps now.

Second, explicitly set -Copt-level=2 instead of using -O

Third, remove unnecessary cruft: we only need to reason about the
shufflevector instruction and the bswap intrinsic.

Author: workingjubilee

tests/codegen/dont-shufflevector-bswaps.rs

@@ -0,0 +1,36 @@
+//@ revisions: OPT2 OPT3
+//@ [OPT2] compile-flags: -Copt-level=2
+//@ [OPT3] compile-flags: -C opt-level=3
+//@ min-llvm-version: 18.1.3
+
+#![crate_type = "lib"]
+#![no_std]
+
+// The code is from https://github.com/rust-lang/rust/issues/122805.
+// Ensure we do not generate the shufflevector instruction
+// to avoid complicating the code.
+// CHECK-LABEL: define{{.*}}void @convert(
+// CHECK-NOT: shufflevector
+// On higher opt levels, this should just be a bswap:
+// OPT3: load <8 x i16>
+// OPT3-NEXT: call <8 x i16> @llvm.bswap
+// OPT3-NEXT: store <8 x i16>
+// OPT3-NEXT: ret void
+#[no_mangle]
+pub fn convert(value: [u16; 8]) -> [u8; 16] {
+    #[cfg(target_endian = "little")]
+    let bswap = u16::to_be;
+    #[cfg(target_endian = "big")]
+    let bswap = u16::to_le;
+    let addr16 = [
+        bswap(value[0]),
+        bswap(value[1]),
+        bswap(value[2]),
+        bswap(value[3]),
+        bswap(value[4]),
+        bswap(value[5]),
+        bswap(value[6]),
+        bswap(value[7]),
+    ];
+    unsafe { core::mem::transmute::<_, [u8; 16]>(addr16) }
+}