[RegisterBankInfo] Uniquely generate OperandsMapping.

[RegisterBankInfo] Uniquely generate OperandsMapping.

This is a step toward statically allocate InstructionMapping. Like the
previous few commits, the goal is to move toward a TableGen'ed like
structure with no dynamic allocation at all.

This should already improve compile time by getting rid of a bunch of
memmove of SmallVectors.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@282643 91177308-0d34-0410-b5e6-96231b3b80d8