[AArch64] Add an assert to aarch64_layout_frame

[AArch64] Add an assert to aarch64_layout_frame

This patch adds an assert that all the individual *_adjust allocations
add up to the full frame size.  With that safety net, it seemed slightly
clearer to use crtl->outgoing_args_size as the final adjustment where
appropriate, to match what's used in the comments.

This is a bit overkill on its own, but I need to add more cases for SVE.

2019-10-16  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64.c (aarch64_layout_frame): Assert
	that all the adjustments add up to the full frame size.
	Use crtl->outgoing_args_size directly as the final adjustment
	where appropriate.

From-SVN: r277060