aarch64: Use an expander for quad-word vec_pack_trunc pattern

aarch64: Use an expander for quad-word vec_pack_trunc pattern

The existing vec_pack_trunc RTL pattern emits an opaque two-
instruction assembly code sequence that prevents proper instruction
scheduling. This commit changes the pattern to an expander that emits
individual xtn and xtn2 instructions.

This commit also consolidates the duplicate truncation patterns.

gcc/ChangeLog:

2021-05-17  Jonathan Wright  <jonathan.wright@arm.com>

	* config/aarch64/aarch64-simd.md (aarch64_simd_vec_pack_trunc_<mode>):
	Remove as duplicate of...
	(aarch64_xtn<mode>): This.
	(aarch64_xtn2<mode>_le): Move position in file.
	(aarch64_xtn2<mode>_be): Move position in file.
	(aarch64_xtn2<mode>): Move position in file.
	(vec_pack_trunc_<mode>): Define as an expander.