[Sparc] Add support for the partial write PSR instruction

Summary:
Partial write %PSR (WRPSR) is a SPARC V8e option that allows WRPSR
instructions to only affect the %PSR.ET field. It is supported by
the GR740 and GR716.

Reviewers: jyknight, venkatra

Subscribers: fedor.sergeev, jrtc27, llvm-commits

Differential Revision: https://reviews.llvm.org/D48644

llvm-svn: 343202