[AArch64][RegisterBankInfo] Provide alternative mappings for G_BITCASTs.

Thanks to this patch, RegBankSelect is able to get rid of some register
bank copies as demonstrated in the test case.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@284094 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed