The artifact combiner searches for the uses of G_MERGE_VALUES for
unmerge/trunc that need further combining. This also needs to handle
the vector merge opcodes the same way. This fixes leaving behind some
pairs I expected to be removed, that were if the legalizer is run a
second time.
Add support in LegalizerHelper for lowering G_SADDSAT etc. either
using add/subtract-with-overflow or using max/min instructions.
Enable this lowering for AMDGPU so it can be tested. The legalization
rules are still approximate and skips out on using the clamp bit to
treat these as legal, which has never been used before. This also
doesn't yet try to deal with expanding SALU cases.
Matt Arsenault