[RISCV] Refactor X60 scheduling model helper classes. NFC. (#151572)

This patch improves the helper classes in the SpacemiT-X60 vector
scheduling model and will be used in follow-up PRs:

There are now two functions to map LMUL to values:
* ConstValueUntilLMULThenDoubleBase: returns BaseValue for LMUL values
before startLMUL, Value for startLMUL, then doubles Value for each
subsequent LMUL. Useful for cases where fractional LMULs have constant
cycles, and integer LMULs double as they increase.
* GetLMULValue: takes an ordered list of LMUL cycles and LMUL and
returns the corresponding cycle. Useful for cases we can't easily cover
with ConstValueUntilLMULThenDoubleBase.

This PR also adds some useful, simplified versions of
ConstValueUntilLMULThenDoubleBase, e.g., ConstValueUntilLMULThenDouble
(when BaseValue == Value), or ConstOneUntilMF4ThenDouble (when cycles
start to double after MF2).

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