cmd/asm/internal/asm: add additional tests for constant loads on riscv64
This improves test coverage around the various constant load edge cases.
Change-Id: Ibafeec78e76d95e9f56b48fa6bd012772bf505c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/652776
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com> Reviewed-by: David Chase <drchase@google.com> Reviewed-by: Mark Ryan <markdryan@rivosinc.com>