1.0.31.22: improved SAP-{REF,SET}-* for x86-64

1.0.31.22: improved SAP-{REF,SET}-* for x86-64

- Don't go through RAX in reffers; zero- or sign-extend the memory
  referenced into the result register directly.  Specialized array
  references already do this.

- Don't go through RAX in setters; use the appropriately-sized low part
  of the value register directly.