[RegisterBankInfo] Provide a target independent helper function to guess

the mapping of an instruction on register bank.

For most instructions, it is possible to guess the mapping of the
instruciton by using the encoding constraints.
It remains instructions without encoding constraints.
For copy-like instructions, we try to propagate the information we get
from the other operands. Otherwise, the target has to give this
information.

llvm-svn: 265703