arm: Auto-vectorization for MVE: vshr

This patch enables MVE vshr instructions for auto-vectorization.  New
MVE patterns are introduced that take a vector of constants as second
operand, all constants being equal.

The existing mve_vshrq_n_<supf><mode> is kept, as it takes a single
immediate as second operand, and is used by arm_mve.h.

The vashr<mode>3 and vlshr<mode>3 expanders are moved fron neon.md to
vec-common.md, updated to rely on the normal expansion scheme to
generate shifts by immediate.

2020-12-03  Christophe Lyon  <christophe.lyon@linaro.org>

	gcc/
	* config/arm/mve.md (mve_vshrq_n_s<mode>_imm): New entry.
	(mve_vshrq_n_u<mode>_imm): Likewise.
	* config/arm/neon.md (vashr<mode>3, vlshr<mode>3): Move to ...
	* config/arm/vec-common.md: ... here.

	gcc/testsuite/
	* gcc.target/arm/simd/mve-vshr.c: Add tests for vshr.