[SPARC] Cleanup handling of the Y/ASR registers.

- Implement copying ASR to/from GPR regs.
- Mark ASRs as non-allocatable, so it won't try to arbitrarily use
  them inappropriately.
- Instead of inserting explicit WRASR/RDASR nodes in the MUL/DIV
  routines, just do normal register copies.
- Also...mark div as using Y, not just writing it.

Added a test case with some code which previously died with an
assertion failure (with -O0), or produced wrong code (otherwise).

(Third time's the charm?)

Differential Revision: http://reviews.llvm.org/D10401

llvm-svn: 241686