[RISCV] Add test case showing suboptimal BUILD_VECTOR lowering

The second test case added here was pointed out to me by @craig.topper
and shows how we "optimize" a two-element BUILD_VECTOR from being one
load from the constant pool to two loads from the constant pool.

The first test case shows that since materialization for the
floating-point +0.0 value is cheap and doesn't involve a load, the
optimization is more clearly beneficial here.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D106962