[RISCV] Obey -riscv-v-fixed-length-vector-elen-max when lowering mask BUILD_VECTORs.

We may not be allowed to use vXiXLen vectors. Consult ELEN to
determine what is allowed. This will become even more important
when Zve32 is added.

Reviewed By: frasercrmck, arcbbb

Differential Revision: https://reviews.llvm.org/D117518