[aarch64][globalisel] Move getValueMapping/getCopyMapping to AArch64GenRegisterBankInfo. NFC.

Summary:
We did lose a little specificity in the assertion messages for the
PartialMappingIdx enumerators in this change but this was necessary to
avoid unnecessary use of 'public:' and we haven't lost anything that
can't be discovered easily in lldb. Once this is tablegen-erated we could
also safely remove the assertions.

Depends on D27976

Reviewers: t.p.northover, ab, rovka, qcolombet

Subscribers: aditya_nandakumar, aemerson, rengolin, vkalintiris, dberris, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D27978

llvm-svn: 291900