[mips] Don't cache IsO32 and IsFP64 in MipsTargetLowering::MipsCC

Summary:
Use a MipsSubtarget reference instead.

No functional change.

Reviewers: vmedic

Reviewed By: vmedic

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5008

llvm-svn: 217434