[llvm] [AMDGPU][AggressiveInstCombine] Narrow 64 bit math to 32 bit if profitable (PR #130577)

[Shilei Tian via llvm-commits]

shiltian wrote:

> > How about sext operations with sign bit is 1?
> 
> Hi @shiltian , there may be problem with sext if I'm not wrong, using the following example:
> 
> ```
> define i64 @narrow_add(i64 noundef %a, i64 noundef %b) {
>   %zext0 = and i64 %a, 1073741824 ; 0x0000000040000000
>   %zext1 = and i64 %b, 1073741824 ; 0x0000000040000000
>   %add = add i64 %zext0, %zext1
>   ret i64 %add
> }
> ```
> 
> So `%zext0` and `%zext1` either going to equal to 0x0000000040000000 or 0 in 64 bit .
> 
> When `%zext0`=`%zext2`=0x40000000 then `%zext0 + %zext2 = 0x0000000080000000`
> 
> If I truncate both `%zext0` and `%zext1` into 32bit, then I have 0x40000000 and truncated add for 32 bit is 0x80000000
> 
> The 31'th bit is 1, with sext this will extend to 0xFFFFFFFF80000000, which is not equals to `%zext0 + %zext2`

I'd add test cases to make sure invalid cases are not combined.

https://github.com/llvm/llvm-project/pull/130577