[RegBankSelect] Introduce MappingCost helper class.

This helper class will be used to represent the cost of mapping an
instruction to a specific register bank.
The particularity of these costs is that they are mostly local, thus the
frequency of the basic block is irrelevant. However, for few
instructions (e.g., phis and terminators), the cost may be non-local and
then, we need to account for the frequency of the involved basic blocks.

This will be used by the greedy mode I am working on.

llvm-svn: 270163