Refactor UpdatePredRedefs and StepForward to avoid duplication. NFC

Note, this is a reapplication of r236515 with a fix to not assert on non-register operands, but instead only handle them until the subsequent commit.  Original commit message follows.

The code was basically the same here already.  Just added an out parameter for a vector of seen defs so that UpdatePredRedefs can call StepForward first, then do its own post processing on the seen defs.

Will be used in the next commit to also handle regmasks.

llvm-svn: 236538