Jit: Extract immediate handling to separate ConstantPropagation class

Restructuring things in this way brings two immediate benefits:

* Code is deduplicated between Jit64 and JitArm64.
* Materializing an immediate value in a register no longer results in us
  forgetting what the immediate value was.

As a more long-term benefit, this lets us also run constant propagation
as part of PPCAnalyst, which could let us do cool stuff in the future
like statically determining whether a conditional branch will be taken.
But I have nothing concrete planned for that right now.

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