aarch64: Remove unspecs from [su]qmovn RTL pattern

aarch64: Remove unspecs from [su]qmovn RTL pattern

Saturating truncation can be expressed using the RTL expressions
ss_truncate and us_truncate. This patch changes the implementation
of the vqmovn_* intrinsics to use these RTL expressions rather than
a pair of unspecs. The redundant unspecs are removed along with their
code iterator.

gcc/ChangeLog:

2021-04-12  Jonathan Wright  <jonathan.wright@arm.com>

	* config/aarch64/aarch64-simd-builtins.def: Modify comment to
	make consistent with updated RTL pattern.
	* config/aarch64/aarch64-simd.md (aarch64_<sur>qmovn<mode>):
	Implement using ss_truncate and us_truncate rather than
	unspecs.
	* config/aarch64/iterators.md: Remove redundant unspecs and
	iterator: UNSPEC_[SU]QXTN and SUQMOVN respectively.