[mips] Remove MipsCC::reservedArgArea() in favour of...

[mips] Remove MipsCC::reservedArgArea() in favour of MipsABIInfo::GetCalleeAllocdArgSizeInBytes(). NFC.

Summary:

Reviewers: theraven, vmedic

Reviewed By: vmedic

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6115

llvm-svn: 221528