cmd/compile: use integer min/max instructions on riscv64

When GORISCV64 enables rva22u64, make use of integer MIN/MINU/MAX/MAXU
instructions in compiler rewrite rules.

Change-Id: I4e7c514516acad03f2869d4c8936f06582cf7ea9
Reviewed-on: https://go-review.googlesource.com/c/go/+/559660


Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>