cmd/compile: cleanups for Node trimming sequence

Suggested during code reviews of last 15 CLs (or so).

Change-Id: If780f6eb47a7a31df133c64d5dcf0eaf04d8447b
Reviewed-on: https://go-review.googlesource.com/10675
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
diff --git a/src/cmd/compile/internal/gc/subr.go b/src/cmd/compile/internal/gc/subr.go
index 627b86d..2ac2fec 100644
--- a/src/cmd/compile/internal/gc/subr.go
+++ b/src/cmd/compile/internal/gc/subr.go
@@ -333,6 +333,7 @@
 		s1.Block = s.Block
 		if s1.Def.Name == nil {
 			Dump("s1def", s1.Def)
+			Fatal("missing Name")
 		}
 		s1.Def.Name.Pack = pack
 		s1.Origpkg = opkg