MSP430: Indiciate that the epilogue_helper insn does not fallthru

This fixes an ICE in rtl_verify_fallthru, at cfgrtl.c:2970
gcc.c-torture/execute/20071210-1.c for -mcpu=msp430 at -O2
and above.

The epilogue_helper insn was treated as a regular insn which will
fallthru, so when a barrier is emitted after it, RTL verification failed
as rtl_verify_fallthru.

gcc/ChangeLog:

2020-04-09  Jozef Lawrynowicz  <jozef.l@mittosystems.com>

	* config/msp430/msp430.c (msp430_expand_epilogue): Use emit_jump_insn
	when to emit the epilogue_helper insn.
	* config/msp430/msp430.md (epilogue_helper): Add a return insn to the
	RTL pattern.

gcc/ChangeLog

+2020-04-09  Jozef Lawrynowicz  <jozef.l@mittosystems.com>
+
+	* config/msp430/msp430.c (msp430_expand_epilogue): Use emit_jump_insn
+	when to emit the epilogue_helper insn.
+	* config/msp430/msp430.md (epilogue_helper): Add a return insn to the
+	RTL pattern.
+
 2020-04-09  Jakub Jelinek  <jakub@redhat.com>
 
 	PR debug/94495

gcc/config/msp430/msp430.c

@@ -2587,7 +2587,7 @@ msp430_expand_epilogue (int is_eh)
 		 && helper_n > 1
 		 && !is_eh)
 	  {
-	    emit_insn (gen_epilogue_helper (GEN_INT (helper_n)));
+	    emit_jump_insn (gen_epilogue_helper (GEN_INT (helper_n)));
 	    return;
 	  }
 	else

gcc/config/msp430/msp430.md

@@ -1130,7 +1130,9 @@
   )
 
 (define_insn "epilogue_helper"
-  [(unspec_volatile [(match_operand 0 "immediate_operand" "i")] UNS_EPILOGUE_HELPER)]
+  [(set (pc)
+        (unspec_volatile [(match_operand 0 "immediate_operand" "i")] UNS_EPILOGUE_HELPER))
+   (return)]
   ""
   "BR%Q0\t#__mspabi_func_epilog_%J0"
   )