[RISCV] Factor out the core part of LMULWriteResMXVariant. NFC (#162347)

LMULWriteResMXVariant is a helper class that makes creating LMUL-aware
`SchedVar` easier. In preparation for later patches that require
  - LMUL- _and_ SEW-aware `SchedVar`
- Assign different processor resources for predicated and non-predicated
variants

I factor out the core logics of LMULWriteResMXVariant into another impl
class, such that it'll be easier to add _"LMULSEWWriteResMXSEWVariant"_
easier later. I also extend this class so that users can customize
processor resources for the non-predicated variant.

Despite these, this patch is still a NFC. I thought it'll be cleaner not
to mix the changes here into later patches.

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