[PHIElimination] Compile time optimization for huge functions.

This is a compile-time optimization for PHIElimination (splitting of critical
edges), which was reported at https://bugs.llvm.org/show_bug.cgi?id=44249. As
discussed there, the way to remedy the slowdowns with huge functions is to
pre-compute the live-in registers for each MBB in an efficient way in
PHIElimination.cpp and then pass that information along to
LiveVariabless::addNewBlock().

In all the huge test programs where this slowdown has been noticable, it has
dissapeared entirely with this patch.

Review: Björn Pettersson, Quentin Colombet.

Differential Revision: https://reviews.llvm.org/D73152