[mips] Generalize the handling of f128 return values to support f128 arguments.

Summary:
This will allow us to handle f128 arguments without duplicating code from
CCState::AnalyzeFormalArguments() or CCState::AnalyzeCallOperands().

No functional change.

Reviewers: vmedic

Reviewed By: vmedic

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5292