cmd/compile: show compiler panics when -d panic=1

Fixes #10683.

Change-Id: I4cce3f298b787c736dbabe544a11a9215bcd3671
Reviewed-on: https://go-review.googlesource.com/10336Reviewed-by: Russ Cox <rsc@golang.org>

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

@@ -36,9 +36,10 @@ var (
 )
 
 var (
-	Debug_wb     int
 	Debug_append int
+	Debug_panic  int
 	Debug_slice  int
+	Debug_wb     int
 )
 
 // Debug arguments.
@@ -53,6 +54,7 @@ var debugtab = []struct {
 	{"disablenil", &Disable_checknil}, // disable nil checks
 	{"gcprog", &Debug_gcprog},         // print dump of GC programs
 	{"nil", &Debug_checknil},          // print information about nil checks
+	{"panic", &Debug_panic},           // do not hide any compiler panic
 	{"slice", &Debug_slice},           // print information about slice compilation
 	{"typeassert", &Debug_typeassert}, // print information about type assertion inlining
 	{"wb", &Debug_wb},                 // print information about write barriers
@@ -89,7 +91,7 @@ func usage() {
 }
 
 func hidePanic() {
-	if nsavederrors+nerrors > 0 {
+	if Debug_panic == 0 && nsavederrors+nerrors > 0 {
 		// If we've already complained about things
 		// in the program, don't bother complaining
 		// about a panic too; let the user clean up