Teach branch folding pass about implicit_def instructions. Unfortunately we can't...

Teach branch folding pass about implicit_def instructions. Unfortunately we can't just eliminate them since register scavenger expects every register use to be defined. However, we can delete them when there are no intra-block uses.  Carefully removing some implicit def's which enable more blocks to be optimized away.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@49461 91177308-0d34-0410-b5e6-96231b3b80d8