[RISCV] Expand scalable-vector truncstores and extloads

Caught in internal testing, these operations are assumed legal by
default, even for scalable vector types. Expand them back into separate
truncations and stores, or loads and extensions.

Also add explicit fixed-length vector tests for these operations, even
though they should have been correct already.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D99654