cmd/compile: mark instantiated generic functions as DUPOK

Unified IR wasn't marking instantiated generic functions as DUPOK,
even though they can appear in multiple compilation units, which
evidently interfered with cmd/link's dead code elimination logic.

Manually confirmed to fix the issue, but non-trivial to test within
$GOROOT/test currently, because it's only reproducible when
cmd/compile is invoked with -p. @rsc is currently investigating
updating test/run.go appropriately, after which I'll revisit writing a
test case.

Fixes #51519.

Change-Id: I74a79ed0ca15b25b826e419714af5ceb6e567012
Reviewed-on: https://go-review.googlesource.com/c/go/+/390956
Trust: Matthew Dempsky <[email protected]>
Run-TryBot: Matthew Dempsky <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: mdempsky

src/cmd/compile/internal/noder/reader.go

@@ -644,6 +644,10 @@ func (pr *pkgReader) objIdx(idx int, implicits, explicits []*types.Type) ir.Node
 		name.Func = ir.NewFunc(r.pos())
 		name.Func.Nname = name
 
+		if r.hasTypeParams() {
+			name.Func.SetDupok(true)
+		}
+
 		rext.funcExt(name)
 		return name
 
@@ -790,6 +794,10 @@ func (r *reader) method(rext *reader) *types.Field {
 	name.Func = ir.NewFunc(r.pos())
 	name.Func.Nname = name
 
+	if r.hasTypeParams() {
+		name.Func.SetDupok(true)
+	}
+
 	rext.funcExt(name)
 
 	meth := types.NewField(name.Func.Pos(), sym, typ)