aarch64: Relax aarch64_sqxtun2<mode> RTL pattern

Use UNSPEC_SQXTUN instead of UNSPEC_SQXTUN2 in aarch64_sqxtun2<mode>
patterns. This allows for more more aggressive combinations and
ultimately better code generation. The now redundant UNSPEC_SQXTUN2
is removed.

gcc/ChangeLog:

2021-05-14  Jonathn Wright  <jonathan.wright@arm.com>

	* config/aarch64/aarch64-simd.md: Use UNSPEC_SQXTUN instead
	of UNSPEC_SQXTUN2.
	* config/aarch64/iterators.md: Remove UNSPEC_SQXTUN2.