[Sparc] Use float register for integer constrained with "f" in inline asm

Summary:
Constraining an integer value to a floating point register using "f"
causes an llvm_unreachable to trigger. This patch allows i32 integers
to be placed in a single precision float register and i64 integers to
be placed in a double precision float register. This matches the behavior
of GCC.

For other types the llvm_unreachable is removed to instead trigger an
error message that points out the offending line.

Reviewers: jyknight, venkatra

Reviewed By: jyknight

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

Differential Revision: https://reviews.llvm.org/D51614

llvm-svn: 349045