Implement more peephole optimizations

[polsys]

Some optimizations that could be applied even in non-optimizing builds, since the peephole optimizer covers up a lot of inefficient lowering:

• Replace "load immediate to %2, do arithmetic with %1 and %2" with "do arithmetic with %1 and imm"
• Replace multiplication and unsigned division with a power of 2 with shifts
• Replace "move %1 to %3, do arithmetic with %2 and %3" with "do arithmetic with %2 and %1" where possible
• Replace "jcc next_block, jmp somewhere" with "jncc somewhere, jmp next_block" so that the latter jump can be elided in codegen