[llvm] [VectorCombine] Add type shrinking and zext propagation for fixed-width vector types (PR #104606)

[Simon Pilgrim via llvm-commits]

================
@@ -2493,6 +2494,91 @@ bool VectorCombine::foldSelectShuffle(Instruction &I, bool FromReduction) {
   return true;
 }
 
+/// Check if instruction depends on ZExt and this ZExt can be moved after the
+/// instruction. Move ZExt if it is profitable
----------------
RKSimon wrote:

Add example folds you're attempting, something like:
```
/// logic(zext(x),y) -> zext(logic(x,trunc(y)))
/// lshr((zext(x),y) -> zext(lshr(x,y))
```

https://github.com/llvm/llvm-project/pull/104606