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.