[AMDGPU] Fix liveness for loops in si-optimize-exec-masking-pre-ra

Follow up to D127894, new liveness update code needs to handle
the case where S_ANDN2 input must be extended through loops when
V_CNDMASK_B32 has been hoisted.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D128800