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

[Daniel Sanders]

David Chisnall requested (in D6117 - [mips] Promote i32 arguments to i64 for the N32/N64 ABI and fix <64-bit structs...) that this be committed and post-commit reviewed instead on the basis that it goes towards fixing a badly broken calling convention and there's nothing obviously wrong with the patches. I'm therefore going to go ahead and commit this.

http://reviews.llvm.org/D6085