The main over-widening patch can introduce quite a few extra casts,
and in many cases those casts simply "tap into" an intermediate
point in an existing extension.  E.g. if we have:

    unsigned char a;
    int ax = (int) a;

and a later operation using ax is shortened to "unsigned short",
we would need:

    unsigned short ax' = (unsigned short) a;

The a->ax extension requires one set of unpacks to get to unsigned
short and another set of unpacks to get to int.  The first set are
then duplicated for ax'.  If both ax and ax' are needed, the a->ax'
extension would end up counting twice during cost calculations.

This patch rewrites the original:

    int ax = (int) a;

into a pattern:

    unsigned short ax' = (unsigned short) a;
    int ax = (int) ax';

so that each extension only counts once.

2018-07-03  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-patterns.c (vect_split_statement): New function.
	(vect_convert_input): Use it to try to split an existing cast.

gcc/testsuite/
	* gcc.dg/vect/vect-over-widen-5.c: Test that the extensions
	get split into two for use by the over-widening pattern.
	* gcc.dg/vect/vect-over-widen-6.c: Likewise.
	* gcc.dg/vect/vect-over-widen-7.c: Likewise.
	* gcc.dg/vect/vect-over-widen-8.c: Likewise.
	* gcc.dg/vect/vect-over-widen-9.c: Likewise.
	* gcc.dg/vect/vect-over-widen-10.c: Likewise.
	* gcc.dg/vect/vect-over-widen-11.c: Likewise.
	* gcc.dg/vect/vect-over-widen-12.c: Likewise.
	* gcc.dg/vect/vect-over-widen-13.c: Likewise.
	* gcc.dg/vect/vect-over-widen-14.c: Likewise.
	* gcc.dg/vect/vect-over-widen-15.c: Likewise.
	* gcc.dg/vect/vect-over-widen-16.c: Likewise.
	* gcc.dg/vect/vect-over-widen-22.c: New test.

From-SVN: r262334