[RISCV] Maintain fixed-length info when optimizing BUILD_VECTORs

I'm not sure how I failed to notice this before, but when optimizing
dominant-element BUILD_VECTORs we would lower via the scalable container type,
which lost us the information about the fixed length of the vector types. By
lowering via the fixed-length type we can preserve that information and
eliminate redundant vsetvli instructions.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D98938