cmd/compile: preserve name association when eliding copies in expand_calls

If v is a Copy of x, we will rewrite v to x. If v has a name
associated to it, let the name associate to x.

Under register ABI, this helps associate in-register Arg values
to the parameters' names. (But does not address all cases.)

Change-Id: I47c779e56c9d0823a88890497e32326bc0290f82
Reviewed-on: https://go-review.googlesource.com/c/go/+/309330
Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/compile/internal/ssa/expand_calls.go

@@ -1449,6 +1449,19 @@ func expandCalls(f *Func) {
 	}
 
 	// Step 6: elide any copies introduced.
+	// Update named values.
+	for _, name := range f.Names {
+		values := f.NamedValues[name]
+		for i, v := range values {
+			if v.Op == OpCopy {
+				a := v.Args[0]
+				for a.Op == OpCopy {
+					a = a.Args[0]
+				}
+				values[i] = a
+			}
+		}
+	}
 	for _, b := range f.Blocks {
 		for _, v := range b.Values {
 			for i, a := range v.Args {