[dev.simd] cmd/compile: remove stores to unread parameters

Currently, we remove stores to local variables that are not read.
We don't do that for arguments. But arguments and locals are
essentially the same. Arguments are passed by value, and are not
expected to be read in the caller's frame. So we can remove the
writes to them as well. One exception is the cgo_unsafe_arg
directive, which makes all the arguments effectively address-taken.
cgo_unsafe_arg implies ABI0, so we just skip ABI0 functions'
arguments.

Change-Id: I8999fc50da6a87f22c1ec23e9a0c15483b6f7df8
Reviewed-on: https://go-review.googlesource.com/c/go/+/705815
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>

test/codegen/stack.go

@@ -168,3 +168,9 @@ func getp1() *[4]int {
 func getp2() *[4]int {
 	return nil
 }
+
+// Store to an argument without read can be removed.
+func storeArg(a [2]int) {
+	// amd64:-`MOVQ\t\$123,.*\.a\+\d+\(SP\)`
+	a[1] = 123
+}