[mips] Removed IsSoftFloat from MipsISelLowering::analyzeCallOperands(). NFC

Summary:
It isn't used anymore.

Depends on D6081

Reviewers: vmedic

Reviewed By: vmedic

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6083

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221463 91177308-0d34-0410-b5e6-96231b3b80d8