[mips] Rework a portion of MipsCC interface. (NFC)

r299766 contained a "conditional move or jump depends on uninitialized value"
fault, identified by valgrind. This occurred as MipsFastISel::finishCall(..)
used CCState over MipsCCState. The latter is required for the TableGen'd calling
convention logic due to reliance on pre-analyzing type information to lower call
results/returns of vectors correctly.

This change modifies the MipsCC AnalyzeCallResult to be useful with both the
SelectionDAG and FastISel lowering logic.

Reviewers: slthakur

Differential Revision: https://reviews.llvm.org/D32004

llvm-svn: 301392