Replace (some) uses of LEA instruction with appopriate MOV/SHL

Consider replacing LEA instructions with the equivalent MOV/SHL instructions.

The LEA instruction is often used to convert a raw (small) integer to a boxed fixnum by multiplying the integer by 4. The LEA instruction is quite long; it can be replaced by the much shorter sequence of MOV (if necessary) ahd SHL. This is also much easier to understand than the LEA instruction. The MOV can often be free due to register renaming.