cmd/compile/internal/gc: rename Fatal to Fatalf

This helps vet see a real issue:

    cmd/internal/gc$ go vet
    gen.go:1223: unreachable code

Fixes #12106.

Change-Id: I720868b07ae6b6d5a4dc6b238baa8c9c889da6d8
Reviewed-on: https://go-review.googlesource.com/14083
Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/src/cmd/compile/internal/gc/syntax.go b/src/cmd/compile/internal/gc/syntax.go
index 7f03a4e..5081ea0 100644
--- a/src/cmd/compile/internal/gc/syntax.go
+++ b/src/cmd/compile/internal/gc/syntax.go
@@ -81,7 +81,7 @@
 	if n.hasVal == -1 {
 		Debug['h'] = 1
 		Dump("have Opt", n)
-		Fatal("have Opt")
+		Fatalf("have Opt")
 	}
 	n.hasVal = +1
 	n.E = v.U
@@ -104,7 +104,7 @@
 	if n.hasVal == +1 {
 		Debug['h'] = 1
 		Dump("have Val", n)
-		Fatal("have Val")
+		Fatalf("have Val")
 	}
 	n.hasVal = -1
 	n.E = x