dd45f44511
x64 backend: add lowerings with load-op-store fusion. These lowerings use the `OP [mem], reg` forms (or in AT&T syntax, `OP %reg, (mem)`) -- i.e., x86 instructions that load from memory, perform an ALU operation, and store the result, all in one instruction. Using these instruction forms, we can merge three CLIF ops together: a load, an arithmetic operation, and a store.