[Sparc] Select correct register class for FP register constraints

Summary: The fX version of floating-point registers only supports
single precision. We need to map the name to dX for doubles and qX
for long doubles if we want getRegForInlineAsmConstraint() to be
able to pick the correct register class.

Reviewers: jyknight, venkatra

Reviewed By: jyknight

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

Differential Revision: https://reviews.llvm.org/D47258

llvm-svn: 333512