[WebAssembly] combine `bitmask` with `setcc <X>, 0, setlt` (#179065)
The rust `simd_bitmask` intrinsic is UB when the lanes of its input are
not either `0` or `!0`, presumably so that the implementation can be
more efficient because it could look at any bit. To get the "mask of
MSB" behavior of webassembly's `bitmask`, we would like to simply first
compare with a zero vector.
```llvm
define i32 @example(<2 x i64> noundef %v) {
entry:
%1 = icmp slt <16 x i8> %v, zeroinitializer
%2 = bitcast <16 x i1> %1 to i16
%3 = zext i16 %2 to i32
ret i32 %3
}
```
On x86_64, this additional comparison optimizes away, but for wasm it
does not.
https://godbolt.org/z/T5sPejocs
This PR adds a new combine, so that instead of emitting
```asm
example:
local.get 0
v128.const 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
i8x16.lt_s
i8x16.bitmask
end_function
```
we just emit
```
example:
local.get 0
i8x16.bitmask
end_function
```
llvm-project
b861a289 - [WebAssembly] combine `bitmask` with `setcc <X>, 0, setlt` (#179065)
