[AMDGPU] Add missing use check in SIOptimizeExecMasking pass.

Whenever a v_cmp, s_and_saveexec instruction sequence shall be
transformed to an equivalent s_mov, v_cmpx sequence, it needs
to be detected if the v_cmp target register is used between
the two instructions as the v_cmp result gets omitted by
using the v_cmpx instruction, resulting in invalid code.

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D122797