AMDGPU: Preserve vcc undef flags when inverting branch

If the branch was on a read-undef of vcc, passes that used
analyzeBranch to invert the branch condition wouldn't preserve
the undef flag resulting in a verifier error.

Fixes verifier failures in a future commit.

Also fix verifier error when inserting copy for vccz
corruption bug.

llvm-svn: 286133