[RegisterBankInfo] Avoid heap allocation in InstructionMapping.

Use SmallVector instead of dynamically allocated arrays for the mapping of the
operands in the InstructionMapping. That way we avoid heap allocation for most
of the cases. Ultimately, we should not have to rely on such tricky, the
instances of InstructionMapping would be TableGen'ed.

This improves the compilation time of the RegBankSelect pass.

llvm-svn: 281955