trivialCode on x86 is the heart of instruction selection for
expressions.  It is definitely a non-trivial in complexity.
To generate correct code it needs to observe preserve several
delicate invariants, but didn't.

-- Recorded in MachCode.lhs the "Rules of the Game"; ie what I think
   the required invariants are.

-- Completely rewrote trivialCode (also shift_code).  I think it should
   handle all cases correctly, and has special treatment for literal
   operands.

-- Updated NOTES file to record issues which need to be resolved
   before x86 nativeGen can be considered ready for public use.