[1/4] aarch64: Convert UABDL and SABDL patterns to standard RTL codes

[1/4] aarch64: Convert UABDL and SABDL patterns to standard RTL codes

This is the first patch in a series to improve the RTL representation of the sum-of-absolute-differences patterns
in the backend. We can use standard RTL codes and remove some unspecs.
For UABDL and SABDL we have a widening of the result so we can represent uabdl (x, y) as (zero_extend (minus (smax (x, y) (smin (x, y)))))
and sabdl (x, y) as (zero_extend (minus (umax (x, y) (umin (x, y))))).
It is important to use zero_extend rather than sign_extend for the sabdl case, as the result of the absolute difference is still a positive unsigned value
(the signedness of the operation refers to the values being diffed, not the absolute value of the difference) that must be zero-extended.

Bootstrapped and tested on aarch64-none-linux-gnu (these intrinsics are reasonably well-covered by the advsimd-intrinsics.exp tests)

gcc/ChangeLog:

	* config/aarch64/aarch64-simd.md (aarch64_<sur>abdl<mode>): Rename to...
	(aarch64_<su>abdl<mode>): ... This.  Use standard RTL ops instead of
	unspec.
	* config/aarch64/aarch64.md (UNSPEC_SABDL, UNSPEC_UABDL): Delete.
	* config/aarch64/iterators.md (ABDL): Delete.
	(sur): Remove handling of UNSPEC_SABDL and UNSPEC_UABDL.