[X86] `detectAVGPattern()`: support basic case of PAVG chaining (PR52131)

As noted in https://github.com/halide/Halide/pull/6302,
we hilariously fail to match PAVG if we even as much
as look at it the wrong way.

In this particular case, the problem stems from the fact that
`PAVG` root (def) is a `trunc`, and leafs (uses) are `zext`'s,
and InstCombine really loves to get rid of both of these,
for example replace them with a bit mask. So we may not have
said `zext`.

Instead of checking for that + type match,
i think we should rely on the actual active type,
as per the knownbits.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D111571