Fix various immediate operands in x86-64 LAP generation.

Fix various immediate operands in x86-64 LAP generation.

Implement an abstraction for yielding an operand representing some
immediate value, either by yielding an actual immediate operand or by
loading an immediate into a temporary register with MOV (which takes
64-bit immediate operands, unlike every other instruction) and by
yielding a reference to the temporary register for the operand.

Use this to simplify LOAD-DISPLACED-REGISTER, and reduce the code it
generates.

Use more appropriate names for procedures that generate code to move
objects into registers (`load') and to move objects into memory
(`store').

Fix various other places that generate LAP with immediates to use the
new abstraction to ensure that they always fit in the relevant
instructions' operands.