When converting phi nodes into select instructions, we shouldn't promote PHI

When converting phi nodes into select instructions, we shouldn't promote PHI
nodes unless we KNOW that we are able to promote all of them.  In this case
promoting the phi to a select is silly because we will always have to do the
call conditionally.  As such, select promotion is actually a pessimization.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@16972 91177308-0d34-0410-b5e6-96231b3b80d8