[RISCV] Change how MMO is rebuilt in...

[RISCV] Change how MMO is rebuilt in lowerFixedLengthVectorLoadToRVV/lowerFixedLengthVectorStoreToRVV (#88811)

Copy the pointer info, flags, alignment, AAInfo, and ranges, but let
getLoad rebuild the MMO using the scalable type used for the the new
load/store. This makes sure the LLT minimum size matches the ContainerVT
minimum size. This is important since vscale_range may have been used to
determine that the fixed vector was the exact size of a scalable vector.

Fixes #88799