[RISCV] Convert more constant splats in tests to splat shorthand. NFC (#87616)

A handy shorthand for specifying the shufflevector(insertelement(poison,
foo, 0), poison, zeroinitializer) splat pattern was introduced in
#74620.

Some of the RISC-V tests were converted over to use this new form in
dbb65dd3, this patch handles the rest
which didn't have any codegen diffs.

This not only converts some constant expressions to the new form, but
also instruction sequences that weren't previously constant expressions
to constant expressions as well. In some cases this affects codegen, but
these have been omitted here and will be handled in a separate PR.