[RISCV] Support fixed-length INSERT_VECTOR_ELT

This patch enables support for lowering INSERT_VECTOR_ELT on
fixed-length vector types. The strategy follows that for scalable vector
types.

This patch also includes a quick fix to prevent the compiler infinitely
looping between lowering BUILD_VECTOR as VECTOR_SHUFFLE and back again.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D97698