[Sparc] Use the IntPair reg class for r constraints with value type f64

Summary: This is how it appears to be handled in GCC and it prevents a
"Unknown mismatch" error in the SelectionDAGBuilder.

Reviewers: venkatra, jyknight, jrtc27

Reviewed By: jyknight, jrtc27

Subscribers: eraman, fedor.sergeev, jrtc27, llvm-commits

Differential Revision: https://reviews.llvm.org/D49218

llvm-svn: 337370