[RISCV] Account for LMUL in memory op costs

Generally, the cost of a memory op will scale with the number of vector registers accessed. Machines might exist which have a narrow memory access than vector register width, but machines with a wider memory access width than vector register width seem unlikely.

I noticed this because we were preferring wide loads + deinterleaves on examples where the cost of a short gather (actually a strided load) would be better. Touching 8 vector registers instead of doing a 4 element gather is not a good tradeoff.

Differential Revision: https://reviews.llvm.org/D147470

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