[globalisel][knownbits] Allow targets to call GISelKnownBits::computeKnownBitsImpl()

Summary:
It seems we missed that the target hook can't query the known-bits for the
inputs to a target instruction. Fix that oversight

Reviewers: aditya_nandakumar

Subscribers: rovka, hiraditya, volkan, Petar.Avramovic, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67380

llvm-svn: 373264