cmd/compile: resolve known outcomes for SLTI/SLTIU on riscv64

cmd/compile: resolve known outcomes for SLTI/SLTIU on riscv64

When SLTI/SLTIU is used with ANDI/ORI, it may be possible to determine the
outcome based on the values of the immediates. Resolve these cases.

Improves code generation for various shift operations.

While here, sort tests by architecture to improve readability and ease
future maintenance.

Change-Id: I87e71e016a0e396a928e7d6389a2df61583dfd8d
Reviewed-on: https://go-review.googlesource.com/c/go/+/428217
Reviewed-by: Wayne Zuo <wdvxdr@golangcn.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Jenny Rakoczy <jenny@golang.org>
Reviewed-by: Jenny Rakoczy <jenny@golang.org>
Run-TryBot: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Jenny Rakoczy <jenny@golang.org>