[mips] Remove MipsCC::analyzeCallOperands in favour of CCState::AnalyzeCallOperands. NFC

Summary:
In addition to the usual f128 workaround, it was also necessary to provide
a means of accessing ArgListEntry::IsFixed.

Reviewers: theraven, vmedic

Reviewed By: vmedic

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6111