R600/SI: Try to use scalar BFE.

Use scalar BFE with constant shift and offset when possible.
This is complicated by the fact that the scalar version packs
the two operands of the vector version into one.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206558 91177308-0d34-0410-b5e6-96231b3b80d8