riscv64: Instruction selection improvements

[afonso360]

👋 Hey,

This PR does two small instruction selection and encoding optimizations.

Use addi for subs

This is one of the regressions that showed up in #7468. That PR changes iadd_imm(x, -k) to isub(x, iconst(k)). We can use the same iadd instruction for both, but we didn't have a rule to do it.

So we now try to negate an Imm12 and if we are able to do that we emit an addi when subtracting.

Try to swap registers when encoding compressed instructions for commutative operations

This is pretty much the same as #7471 but for all of the remaining compressed commutative instructions.

It allows us to emit these instructions as compressed more often.