[AMDGPU] Optimize S_CBRANCH_VCC[N]Z -> S_CBRANCH_EXEC[N]Z

Sometimes after basic block placement we end up with a code like:

  sreg = s_mov_b64 -1
  vcc = s_and_b64 exec, sreg
  s_cbranch_vccz

This happens as a join of a block assigning -1 to a saved mask and
another block which consumes that saved mask with s_and_b64 and a
branch.

This is essentially a single s_cbranch_execz instruction when moved
into a single new basic block.

Differential Revision: https://reviews.llvm.org/D54164

llvm-svn: 346690