[PPC] Fix suspicious AltiVec VAVG patterns (#176891)
The existing ((X+Y+1)>>1) patterns didn't correct handle overflow, like
the VAVG instructions would
Remove the old patterns and correctly mark the altivec VAVGS/VAVGU
patterns as matching the ISD::AVGCEIL opcodes - the generic DAG folds
will handle everything else
I've updated the vavg.ll tests to correct match ISD::AVGCEILS/U patterns
and added the old tests as negative "overflow" patterns that shouldn't
fold to VAVG instructions
Fixes #174718
