[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217434 91177308-0d34-0410-b5e6-96231b3b80d8