[llvm] [Mips] Fix missing sign extension in expansion of sub-word atomic max (PR #77072)

[Quentin Dian via llvm-commits]

DianQK wrote:

Clarify: I don't have any knowledge about MIPS, and the following is only confirmed from the output logs. Please correct me if there is any mistake. :)

TL;DR: All issues have been fixed in LLVM 17. This appears to be an incorrect patch; we might consider reverting it. But I hope @yingopq and others can come to confirm this.

Consider the following two pieces of code:

```llvm
; v1.ll https://github.com/rust-lang/rust/issues/100650
define i8 @test(ptr %arg) {
  %i6 = atomicrmw max ptr %arg, i8 -25 seq_cst, align 1
  %i7 = load atomic i8, ptr %arg seq_cst, align 1
  ret i8 %i7
}

define i32 @main() {
  %i = alloca i8, align 1
  store i8 30, ptr %i, align 1
  %1 = call i8 @test(ptr %i)
  %2 = sext i8 %1 to i32
  ret i32 %2
}
```

and

```llvm
; v2.ll https://github.com/rust-lang/rust/issues/123772
define i8 @test(ptr %arg) {
  %i6 = atomicrmw max ptr %arg, i8 42 seq_cst, align 1
  ret i8 %i6
}

define i32 @main() {
  %i = alloca i8, align 1
  store i8 23, ptr %i, align 1
  %1 = call signext i8 @test(ptr %i)
  %2 = sext i8 %1 to i32
  ret i32 %2
}
```

I can confirm that the return value of `v2.ll` from llvmorg-17-init to 18.1.2 is always 23. However, starting from 18.1.3, it changes to 0.
For `v1.ll`, starting with the commit c65b4d64d4b09795fe237b62a4226121c5b13248, the return value changes from 231 to 30. However, starting from 18.1.3, it also changes to 0.

This is my verification script:

```bash
LLC=path/llc
MIPSEL_GCC=path/mipsel-unknown-linux-gnu-gcc

$LLC -march=mipsel -O0 -mcpu=mips32 -verify-machineinstrs v1.ll -filetype=obj -o v1.o
$MIPSEL_GCC -o v1 v1.o
qemu-mipsel v1
mipsel=$?
$LLC -O0 -march=x86-64 v1.ll -filetype=obj -o v1.o
clang -o v1 v1.o
./v1
x86=$?
echo "v1: $mipsel == $x86"

$LLC -march=mipsel -O0 -mcpu=mips32 -verify-machineinstrs v2.ll -filetype=obj -o v2.o
$MIPSEL_GCC -o v2 v2.o
qemu-mipsel v2
mipsel=$?
$LLC -O0 -march=x86-64 v2.ll -filetype=obj -o v2.o
clang -o v2 v2.o
./v2
x86=$?
if [ "$mipsel" != "$x86" ]; then
  bad=$((bad + 1))
fi
echo "v2: $mipsel == $x86"
```

https://github.com/llvm/llvm-project/pull/77072