ExecutionDepsFix refactoring:

- Changing DenseMap<MBB*, LiveReg*> to SmallVector<LiveReg*>
- Now the MBB number will be the index of LiveReg in the vector.
- Adding asserts

This patch is NFC.

This is the one of multiple patches that fix bugzilla https://bugs.llvm.org/show_bug.cgi?id=33869
Most of the patches are intended at refactoring the existent code.

Additional relevant reviews:
https://reviews.llvm.org/D40330
https://reviews.llvm.org/D40332
https://reviews.llvm.org/D40333
https://reviews.llvm.org/D40334

Differential Revision: https://reviews.llvm.org/D40331

Change-Id: If4a3f141693d0361ddb292432337dbb63a1e69ee
llvm-svn: 323089