[mips] Remove inverted predicates from MipsSubtarget that were only used by MipsCallingConv.td

Summary: No functional change

Reviewers: echristo, vmedic

Reviewed By: echristo, vmedic

Subscribers: echristo, llvm-commits

Differential Revision: http://reviews.llvm.org/D5266

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217494 91177308-0d34-0410-b5e6-96231b3b80d8