test/codegen: add shift tests for RISCV64

Add tests for shift by constant, masked shifts and bounded shifts. While here,
sort tests by architecture and keep order of tests consistent (lsh, rshU, rsh).

Change-Id: I512d64196f34df9cb2884e8c0f6adcf9dd88b0fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/351289


Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Michael Munday <mike.munday@lowrisc.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>