[SOL] Revamp peephole pass for explicit sign extension

[LucasSte]

PR #116 commented out the peephole pass that used to remove unnecessary sign extensions when the ALU32 target feature was enabled. This PR revamps the implementation to remove redundant MOV64 that serves as zero extension.

The reason this works is the following:

1. Every ALU32 instructions, except for mov32 reg, reg, zero extends values. This also includes mov32 reg, imm
2. Truncating from a 64-bit register will always require a bit mask.
3. We don't emit mov32 reg, reg for copying 32-bit values.
4. It is safe to assume that every value in a 32-bit register is always zero extended, so the MOV64 is unnecessary (it only acts as a copy).