[RegisterBankInfo] Avoid heap allocation in most cases.

The OperandsMapper class is used heavy in RegBankSelect and each
instantiation triggered a heap allocation for the array of operands.
Instead, use a SmallVector with a big enough size such that most of the
cases do not have to use dynamically allocated memory.

This improves the compile time of the RegBankSelect pass.

llvm-svn: 281916