for the function
* Registers aren't necessarily sequential wrt their enums, don't rely on it
  when emitting function arguments into sequential registers
* Remove X86-specific comments about AL/BL/AH/BH/EDX/etc
* Add an abort() for an unimplemented signed right shift
* The src operand for a GEP was never emitted!  Fixed.
* We can skip zero-valued GEP indices as they are no-ops.

"Hello, World!" now works.

llvm-svn: 14505