[llvm] [InstCombine] Simplify zext(sub(0, trunc(x))) -> and(sub(0, x), mask) (Fixes #165306) (PR #167101)

[V S Susi Krishna via llvm-commits]

Susikrishna wrote:

You were right about the scalar case, it worked when I kept the datalayout.
Like RKSimon mentioned, when I run the test (even with the datalayout), my output shows that:

    @foo (which uses <2 x i64>)

    @bar (which uses <4 x i64>)

    @baz (which uses <8 x i64>) ...are all still not being simplified.

I think I found the reason for the difference. I was looking at the shouldChangeType function, and the Type* version has this check at the very top:
C++

```
bool InstCombinerImpl::shouldChangeType(Type *From, Type *To) const {
  if (!From->isIntegerTy() || !To->isIntegerTy())
    return false; 
...
}
```

This check returns false for all vector types, so the canEvaluateZExtd path is never reached for them (but it is reached for the scalar i6 type).

https://github.com/llvm/llvm-project/pull/167101