Require equal shift amounts for IFN_DIV_POW2

Require equal shift amounts for IFN_DIV_POW2

IFN_DIV_POW2 currently requires all elements to be shifted by the
same amount, in a similar way as for WIDEN_LSHIFT_EXPR.  This patch
enforces that when building the SLP tree.

If in future targets want to support IFN_DIV_POW2 without this
restriction, we'll probably need the kind of vector-vector/
vector-scalar split that we already have for normal shifts.

2020-01-06  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-slp.c (vect_build_slp_tree_1): Require all shifts
	in an IFN_DIV_POW2 node to be equal.

gcc/testsuite/
	* gcc.target/aarch64/sve/asrdiv_1.c: Remove trailing %s.
	* gcc.target/aarch64/sve/asrdiv_2.c: New test.
	* gcc.target/aarch64/sve/asrdiv_3.c: Likewise.

From-SVN: r279908