[RISCV] Further optimize BUILD_VECTORs with repeated elements

This patch builds upon the initial BUILD_VECTOR work introduced in
D98700. It further optimizes the lowering of BUILD_VECTOR by using
VSELECT operations to effectively insert repeated elements into the
vector with relatively few instructions. This allows us to optimize more
BUILD_VECTORs without significantly increasing the size of the generated
code.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D98969