Revert "r223440 - Consider subregs when calling MI::registerDefIsDead for phys deps"

Reverting this because, while it fixes the problem in the reduced test case, it
does not fix the problem in the full test case from the bug report.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223442 91177308-0d34-0410-b5e6-96231b3b80d8