CodeGen: Cleanup regmask construction; NFC

- Avoid duplication of regmask size calculation.
- Simplify allocateRegisterMask() call.
- Rename allocateRegisterMask() to allocateRegMask() to be consistent
  with naming in MachineOperand.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@337986 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed