[DAGCombiner] Add combine avg from shifts (#113909)

This teaches dagcombiner to fold:
`(asr (add nsw x, y), 1) -> (avgfloors x, y)`
`(lsr (add nuw x, y), 1) -> (avgflooru x, y)`

as well the combine them to a ceil variant:
`(avgfloors (add nsw x, y), 1) -> (avgceils x, y)` 
`(avgflooru (add nuw x, y), 1) -> (avgceilu x, y)`

iff valid for the target.

Removes some of the ARM MVE patterns that are now dead code.
It adds the avg opcodes to `IsQRMVEInstruction` as to preserve the
immediate splatting as before.

0 files changed, 0 insertions, 0 deletions