[WebAssembly] vi8 mul cost modelling. (#175177)
We've already optimised these, so update the cost model to reflect it.
And skip the isBeforeLegalize check when lowering i8 muls, because it
then misses the cases where, say v32i8, has been type legalised into 2x
v16i8.
Also explicitly disable memory interleaving for any factor other than
two or four.
