[AMDGPU] Extend pre-emit peephole to redundantly masked VCC

Extend pre-emit peephole for S_CBRANCH_VCC[N]Z to eliminate
redundant S_AND operations against EXEC for V_CMP results in VCC.
These occur after after register allocation when VCC has been
selected as the comparison destination.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D120202