MSP430: Indiciate that the epilogue_helper insn does not fallthru

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.