ExecutionDepsFix refactoring:

- Changing LiveRegs to be a vector

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: I9cdd364bd7bf2a0bf61ea41a48d4bd310ec3bce4
llvm-svn: 323090