[RISCV] Sink some encoding related lets into class/def bodies. NFC (#179544)

Rather than using lets around classes/defs, override them in the class
def/body.

Some of these lets were around single class/def were I thought it was
better to be inside. Some were around multiple unrelated classes where
it seemed better not to link their encodings like that.

For vmv, I added a multiclass to better encapsulate them but still kept
the let scope to avoid repetition. The encodings are closely related
enough that I thought this was ok.

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