[LSR] Split the -lsr-term-fold transformation into it's own pass (#104234)

This transformation doesn't actually use any of the internal state of
LSR and recomputes all information from SCEV.  Splitting it out makes
it easier to test.
    
Note that long term I would like to write a version of this transform
which *is* integrated with LSR's solver, but if that happens, we'll
just delete the extra pass.
    
Integration wise, I switched from using TTI to using a pass configuration
variable.  This seems slightly more idiomatic, and means we don't run
the extra logic on any target other than RISCV.

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