[RegisterBankInfo] Add a way to record what register bank covers a

specific type.

This will be used to find the default mapping of the instruction.
Also, this information is recorded, instead of computed, because it is
expensive from a type to know which register bank maps it.
Indeed, we need to iterate through all the register classes of all the
register banks to find the one that maps the given type.

llvm-svn: 265736