Sparc: Prefer reg+reg address encoding when only one register used.

Reg+%g0 is preferred to Reg+imm0 by the manual, and is what GCC produces.

Futhermore, reg+imm is invalid for the (not yet supported) "alternate
address space" instructions.

Differential Revision: http://reviews.llvm.org/D8753

llvm-svn: 236107