MSP430: Fix CFA generation during function epilogues

MSP430: Fix CFA generation during function epilogues

There is no CFA information generated for instructions that manipulate the
stack during function epilogues. This means a debugger cannot determine the
position of variables on the stack whilst the epilogue is in progress.

This can cause the debugger to give erroneous information when printing a
backtrace whilst stepping through the epilogue, or cause software watchpoints
set on stack variables to become invalidated after a function epilogue
is executed.

The patch fixes this by marking stack manipulation insns as
frame_related, and adding reg_note RTXs to stack pop instructions in the
epilogue.

gcc/ChangeLog:

	* config/msp430/msp430.c (increment_stack): Mark insns which increment
	the stack as frame_related.
	(msp430_expand_prologue): Add comments.
	(msp430_expand_epilogue): Mark insns which decrement
	the stack as frame_related.
	Add reg_note to stack pop insns describing position of register
	variables on the stack.