[X86] Failure to produce chained PAVG's

[LebedevRI]

Bugzilla Link	52131
Version	trunk
OS	Linux
CC	@abadams,@topperc,@RKSimon,@phoebewang,@rotateright

Extended Description

As noted in halide/Halide#6302

https://godbolt.org/z/9nY77r35q

Haven't really looked at what is actually going wrong yet,
just documenting.

[LebedevRI]

For future self-reference: the actual preconditions on that transform should be
https://alive2.llvm.org/ce/z/SjCKvL
i.e. the inputs must be effectively i8, and both source add's must be NUW.

[RKSimon]

This was fixed by https://reviews.llvm.org/D111571 / 5f4f5da - any additional handling can be added after #92096