LivePhysRegs: Skip reserved regs in computeLiveIns; NFCI

Re-commit r303937 + r303949 as they were not the cause for the build
failures.

We do not track liveness of reserved registers so adding them to the
liveins list in computeLiveIns() was completely unnecessary.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@303970 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed