[mips] Fix fcopysign for MIPS-IV and add the test.

Summary:
This was another incorrect use of hasMips64() vs isGP64bit().

Depends on D3344

Reviewers: matheusalmeida, vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D3347

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206187 91177308-0d34-0410-b5e6-96231b3b80d8