[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

llvm-svn: 206187