[mips] Refactor saved-registers bitmask creation in MipsAsmPrinter::printSavedRegsBitmask. NFC.

Summary:
Make the code more readable by fusing the for-loops together and explicitly checking for each register class.

Also, this version is more straightforward because it doesn't assume that FPU registers always come before CPU registers in the CalleeSavedInfo vector.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8033

llvm-svn: 234475