[RISCV] Use SHL_ADD in remaining strength reduce cases for MUL (#89789)

The interesting bit is the zext folding. This is the first case where we
end up with a profitable fold of shNadd (zext x), y to shNadd.uw x, y.
See zext_mul68 from rv64zba.ll.

The test differences are cases where we can legally fold (only because
there's no one use check). These are not profitable or harmful, but we
can't a oneuse check without breaking the zext_mul68 case.

Note that XTHeadBa doesn't appear to have the equivalent patterns so
this only shows up in Zba.

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