[mips] Removed MipsISelLowering::analyzeFormalArguments() in favour of CCState::AnalyzeFormalArguments()

As with returns, we must be able to identify f128 arguments despite them
being lowered away. We do this with a pre-analyze step that builds a
vector and then we use this vector from the tablegen-erated code.
[mips] Removed IsSoftFloat from MipsISelLowering::analyzeCallOperands()

It isn't used anymore.
[mips] Removed IsVarArg from MipsISelLowering::analyzeCallOperands()

CCState objects already carry this information in their isVarArg() method.
[mips] Move SpecialCallingConv to MipsCCState and use it from tablegen-erated code

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().
[mips] Remove MipsCC::analyzeCallOperands in favour of CCState::AnalyzeCallOperands

In addition to the usual f128 workaround, it was also necessary to provide
a means of accessing ArgListEntry::IsFixed.
[mips] Remove MipsCC::getRegVT()

It's no longer used.
[mips] Remove remaining use of MipsCC::intArgRegs() in favour of MipsABIInfo::GetByValArgRegs() and MipsABIInfo::GetVarArgRegs()
[mips] Move MipsCCState to a separate file and clang-formatted it.
[mips] Remove MipsCC::reservedArgArea() in favour of MipsABIInfo::GetCalleeAllocdArgSizeInBytes
[mips] Removed the remainder of MipsCC

One of the calls to AllocateStack (the one in LowerCall) doesn't look like
it should be there but it was there before and removing it breaks the
frame size calculation.