PR68577: Handle narrowing for vector popcount, etc.

PR68577: Handle narrowing for vector popcount, etc.

This patch adds support for simple cases where a vector
internal function returns wider results than the scalar
equivalent.  It punts on other cases.

Tested on powerpc64-linux-gnu and x86_64-linux-gnu.

gcc/
	PR tree-optimization/68577
	* tree-vect-stmts.c (simple_integer_narrowing): New function.
	(vectorizable_call): Restrict internal function handling
	to NONE and NARROW cases, using simple_integer_narrowing
	to test for the latter.  Add cost of narrowing operation
	and insert it where necessary.

gcc/testsuite/
	PR tree-optimization/68577
	* gcc.dg/vect/pr68577.c: New test.

From-SVN: r231131