cmd/compile/internal/gc: don't iterate over field list twice

cmd/compile/internal/gc: don't iterate over field list twice

In tostruct0 and tofunargs we take a list of nodes, transform them into
a slice of Fields, set the fields on a type, then use the IterFields
iterator to iterate over the list again to see if any of them are
broken.

As we know the slice of fielde-we just created it-we can combine these two
interations into one pass over the fields.

Change-Id: I8b04c90fb32fd6c3b1752cfc607128a634ee06c5
Reviewed-on: https://go-review.googlesource.com/21350
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>