[Sparc] Use synthetic instruction clr to zero register instead of sethi

Using `clr reg`/`mov %g0, reg`/`or %g0, %g0, reg` to zero a register
looks much better than `sethi 0, reg`.

Reviewers: jyknight, venkatra

Reviewed By: jyknight

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

Differential Revision: https://reviews.llvm.org/D45810

llvm-svn: 330396