Remove TargetInstrInfo::canFoldMemoryOperand

canFoldMemoryOperand is not actually used anywhere in the codebase - all existing users instead call foldMemoryOperand directly when they wish to fold and can correctly deduce what they need from the return value. 

This patch removes the canFoldMemoryOperand base function and the target implementations; only x86 had a real (bit-rotted) implementation, although AMDGPU had a preparatory stub that had never needed to be completed.

Differential Revision: http://reviews.llvm.org/D11331

llvm-svn: 242638