[AArch64] Improve poly_int handling in aarch64_layout_frame

[AArch64] Improve poly_int handling in aarch64_layout_frame

I'd used known_lt when converting these conditions to poly_int,
but on reflection that was a bad choice.  The code isn't just
doing a range check; it specifically needs constants that will
fit in a certain encoding.

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

gcc/
	* config/aarch64/aarch64.c (aarch64_layout_frame): Use is_constant
	rather than known_lt when choosing frame layouts.

From-SVN: r277061