[MIPS GlobalISel] Refactor MipsRegisterBankInfo [NFC]

Check if size of operand LLT matches sizes of available register banks
before inspecting the opcode in order to reduce number of checks.
Factor commonly used pieces of code into functions.

Differential Revision: https://reviews.llvm.org/D68866


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@374870 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed