[mips] Return {f128} correctly for N32/N64.

Summary:
According to the ABI documentation, f128 and {f128} should both be returned
in $f0 and $f2. However, this doesn't match GCC's behaviour which is to
return f128 in $f0 and $f2, but {f128} in $f0 and $f1.

Reviewers: vmedic

Reviewed By: vmedic

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5578

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219196 91177308-0d34-0410-b5e6-96231b3b80d8