[AMDGPU] Simplify negated condition

Optimize sequence:

  %sel = V_CNDMASK_B32_e64 0, 1, %cc
  %cmp = V_CMP_NE_U32 1, %1
  $vcc = S_AND_B64 $exec, %cmp
  S_CBRANCH_VCC[N]Z
=>
  $vcc = S_ANDN2_B64 $exec, %cc
  S_CBRANCH_VCC[N]Z

It is the negation pattern inserted by DAGCombiner::visitBRCOND() in the
rebuildSetCC().

Differential Revision: https://reviews.llvm.org/D55402

llvm-svn: 349003