Move variable down to use

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219867 91177308-0d34-0410-b5e6-96231b3b80d8
R600: Remove unnecessary part of computeKnownBitsForTargetNode

Zero-width BFEs are combined away already, so there's no point in
handling them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219868 91177308-0d34-0410-b5e6-96231b3b80d8