cmd/compile: ignore receiver name when checking duplicate methods

In golang.org/cl/20602, I changed the semantics of Eqtype to stop
checking the receiver parameters for type equality, and pushed this
responsibility to addmethod (the only Eqtype caller that cared).
However, I accidentally made the check stricter by making it start
requiring that receiver names were identical.

In general, this is a non-problem because the receiver names in export
data will always match the original source. But running
GO_GCFLAGS=-newexport ./all.bash at one point tries to load both old
and new format export data for package sync, which reveals the
problem. (See golang.org/issue/14877 for details.)

Easy fix: just check the receiver type for type equality in addmethod,
instead of the entire receiver parameter list.

Fixes #14877.

Change-Id: If10b79f66ba58a1b7774622b4fbad1916aba32f1
Reviewed-on: https://go-review.googlesource.com/20906
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/dcl.go

@@ -1264,7 +1264,7 @@ func addmethod(msym *Sym, t *Type, tpkg *Pkg, local, nointerface bool) {
 		}
 		// Eqtype only checks that incoming and result parameters match,
 		// so explicitly check that the receiver parameters match too.
-		if !Eqtype(t, f.Type) || !Eqtype(t.Recvs(), f.Type.Recvs()) {
+		if !Eqtype(t, f.Type) || !Eqtype(t.Recv().Type, f.Type.Recv().Type) {
 			Yyerror("method redeclared: %v.%v\n\t%v\n\t%v", pa, msym, f.Type, t)
 		}
 		return