[AMDGPU] Fix hang caused by VS_CNT handling at calls (#78318)

Fix a potential hang introduced by #77439 and #77935. This line:

  setScoreUB(VS_CNT, getScoreLB(VS_CNT) + getWaitCountMax(VS_CNT));

could potentialy set UB lower than it was before, which confused
SIInsertWaitcnts's fixed point algorithm.

This was only triggered a STORE instruction with an implicit-def, which
seems odd but apparently happens for some spills.