[AMDGPU] Revise handling of preexisting waitcnt

Preexisting waitcnt may not update the scoreboard if the instruction
being examined needed to wait on fewer counters than what was encoded in
the old waitcnt instruction. Fixing this results in the elimination of
some redudnat waitcnt.

These changes also enable combining consecutive waitcnt into a single
S_WAITCNT or S_WAITCNT_VSCNT instruction.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D100281