Fix a fairly major performance problem. If a PHI node had a constant as

Fix a fairly major performance problem.  If a PHI node had a constant as
an incoming value from a block, the selector would evaluate the constant
at the TOP of the block instead of at the end of the block.  This made the
live range for the constant span the entire block, increasing register
pressure needlessly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@12542 91177308-0d34-0410-b5e6-96231b3b80d8