AMDGPU: Clear kill flags when optimizing vcmp save exec sequence

It was causing bad machine code for several blender scenes:
  *** Bad machine code: Using an undefined physical register ***
  - function:    kernel_holdout_emission_blurring_pathtermination_ao
  - basic block: %bb.28 if.end40.i (0x7f84861a2320)
  - instruction: V_CMPX_EQ_U32_nosdst_e64 0, $vgpr3, implicit-def $exec, implicit $exec
  - operand 1:   $vgpr3

Differential Revision: https://reviews.llvm.org/D127768