[RegisterBankInfo] Uniquely allocate instruction mapping.

[RegisterBankInfo] Uniquely allocate instruction mapping.

This is a step toward having statically allocated instruciton mapping.
We are going to tablegen them eventually, so let us reflect that in
the API.

NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@302316 91177308-0d34-0410-b5e6-96231b3b80d8