Implement SLP of internal functions

Implement SLP of internal functions

SLP of calls was previously restricted to built-in functions.
This patch extends it to internal functions.

2018-07-12  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* internal-fn.h (vectorizable_internal_fn_p): New function.
	* tree-vect-slp.c (compatible_calls_p): Likewise.
	(vect_build_slp_tree_1): Remove nops argument.  Handle calls
	to internal functions.
	(vect_build_slp_tree_2): Update call to vect_build_slp_tree_1.

gcc/testsuite/
	* gcc.dg/vect/vect-cond-arith-6.c: New test.
	* gcc.target/aarch64/sve/cond_arith_4.c: Likewise.
	* gcc.target/aarch64/sve/cond_arith_4_run.c: Likewise.
	* gcc.target/aarch64/sve/cond_arith_5.c: Likewise.
	* gcc.target/aarch64/sve/cond_arith_5_run.c: Likewise.
	* gcc.target/aarch64/sve/slp_14.c: Likewise.
	* gcc.target/aarch64/sve/slp_14_run.c: Likewise.

From-SVN: r262590