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

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221517 91177308-0d34-0410-b5e6-96231b3b80d8