cmd/internal/obj/riscv: factor out shift constant code
Move the code that tests to see if a constant can be represented by a
32 bit signed integer and a logical left shift. This reduces duplication
and increases readability. Also add test coverage now that this is an
independent function.
Change-Id: Id25395b1380b00cf5b69ca201b7715ef84f7ade6
Reviewed-on: https://go-review.googlesource.com/c/go/+/652777
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com> Reviewed-by: David Chase <drchase@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com>