cmd/gc: correct liveness for func ending in panic

The registerization code needs the function to end in a RET,
even if that RET is actually unreachable.

The liveness code needs to avoid such unreachable RETs.
It had a special case for final RET after JMP, but no case
for final RET after UNDEF. Instead of expanding the special
cases, let fixjmp - which already knows what is and is not
reachable definitively - mark the unreachable RET so that
the liveness code can identify it.

TBR=iant
CC=golang-codereviews
https://golang.org/cl/63680043

test/live.go

@@ -113,3 +113,11 @@ func f9() bool {
 	x := i9
 	return x != 99
 }
+
+// liveness formerly confused by UNDEF followed by RET,
+// leading to "live at entry to f10: ~r1" (unnamed result).
+
+func f10() string {
+	panic(1)
+}
+