[mips] Move SpecialCallingConv to MipsCCState and use it from tablegen-erated code. NFC

Summary:
In the long run, it should probably become a calling convention in its own
right but for now just move it out of
MipsISelLowering::analyzeCallOperands() so that we can drop this function
in favour of CCState::AnalyzeCallOperands().

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6085