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.