R600: Move fabs/fneg/sel folding logic into PostProcessIsel

This move makes possible to correctly handle multiples instructions
from a single pattern.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190643 91177308-0d34-0410-b5e6-96231b3b80d8
R600: Move code handling literal folding into R600ISelLowering.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190644 91177308-0d34-0410-b5e6-96231b3b80d8
R600: Move clamp handling code to R600IselLowering.cpp

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190645 91177308-0d34-0410-b5e6-96231b3b80d8