Place new basic blocks immediately after their predecessor when splitting

Place new basic blocks immediately after their predecessor when splitting
critical edges in PHIElimination.

This has a huge impact on regalloc performance, and we recover almost all of
the 10% compile time regression that edge splitting introduced.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@89381 91177308-0d34-0410-b5e6-96231b3b80d8