[RISCV] Truncate constants to EltSize when combine store of BUILD_VECTOR

The constants can be with larger bit width, so we need to truncate
them to EltSize or we will exceed the width of fixed-length vector.

Fixes #64588

Reviewed By: luke, craig.topper, bjope, michaelmaitland

Differential Revision: https://reviews.llvm.org/D157603