[LSR / TTI / SystemZ] Eliminate TargetTransformInfo::isFoldableMemAccess()

isLegalAddressingMode() has recently gained the extra optional Instruction*
parameter, and therefore it can now do the job that previously only
isFoldableMemAccess() could do.

The SystemZ implementation of isLegalAddressingMode() has gained the
functionality of checking for offsets, which used to be done with
isFoldableMemAccess().

The isFoldableMemAccess() hook has been removed everywhere.

Review: Quentin Colombet, Ulrich Weigand
https://reviews.llvm.org/D35933

llvm-svn: 310463