arm: Auto-vectorization for MVE: vclz

arm: Auto-vectorization for MVE: vclz

This patch adds support for auto-vectorization of clz for MVE.

It does so by removing the unspec from mve_vclzq_<supf><mode> and uses
'clz' instead. It moves to neon_vclz<mode> expander from neon.md to
vec-common.md and renames it into the standard name clz<mode>2.

2021-06-09  Christophe Lyon  <christophe.lyon@linaro.org>

	gcc/
	* config/arm/iterators.md (<supf>): Remove VCLZQ_U, VCLZQ_S.
	(VCLZQ): Remove.
	* config/arm/mve.md (mve_vclzq_<supf><mode>): Add '@' prefix,
	remove <supf> iterator.
	(mve_vclzq_u<mode>): New.
	* config/arm/neon.md (clz<mode>2): Rename to neon_vclz<mode>.
	(neon_vclz<mode): Move to ...
	* config/arm/unspecs.md (VCLZQ_U, VCLZQ_S): Remove.
	* config/arm/vec-common.md: ... here. Add support for MVE.

	gcc/testsuite/
	* gcc.target/arm/simd/mve-vclz.c: New test.