[LiveVariables] Improve isLiveOut runtime performances. NFC.

On large goto table based interpreters, where phi nodes can have (very) large
fan-ins, isLiveOut exhibited poor performances: about 40% of the full
codegen time was spent in PHIElim, sorting MachineBasicBlock addresses.

This patch improve the performances for such cases, and does not show
compile time regressions on the LNT, at bootstrap (llvm+clang+lldb) or
any other benchmarks we have in-house.

llvm-svn: 239510