[RISCV] Eliminate getVLENFactoredAmount and expose muladd [nfc] (#87881)

This restructures the code to make the fact that most of
getVLENFactoredAmount is just a generic multiply w/immediate more
obvious and prepare for a couple of upcoming enhancements to this code.

Note that I plan to switch mulImm to early return, but decided I'd do
that as a separate commit to keep this diff readable.

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Co-authored-by: Luke Lau <luke_lau@icloud.com>