[ARM] Remove redundant computeKnownBits helper.

Move the BFI logic to computeKnownBitsForTargetNode, and delete
the redundant CMOV logic.

This is intended as a cleanup, but it's probably possible to construct
a case where moving the BFI logic allows more combines.

Differential Revision: https://reviews.llvm.org/D31795

llvm-svn: 300752