match.pd: Canonicalize (x + (x << cst)) into (x * cst2) [PR94800]

match.pd: Canonicalize (x + (x << cst)) into (x * cst2) [PR94800]

The popcount* testcases show yet another creative way to write popcount,
but rather than adjusting the popcount matcher to deal with it, I think
we just should canonicalize those (X + (X << C) to X * (1 + (1 << C))
and (X << C1) + (X << C2) to X * ((1 << C1) + (1 << C2)), because for
multiplication we already have simplification rules that can handle nested
multiplication (X * CST1 * CST2), while the the shifts and adds we have
nothing like that.  And user could have written the multiplication anyway,
so if we don't emit the fastest or smallest code for the multiplication by
constant, we should improve that.  At least on the testcases seems the
emitted code is reasonable according to cost, except that perhaps we could
in some cases try to improve expansion of vector multiplication by
uniform constant.

2020-05-05  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/94800
	* match.pd (X + (X << C) to X * (1 + (1 << C)),
	(X << C1) + (X << C2) to X * ((1 << C1) + (1 << C2))): New
	canonicalizations.

	* gcc.dg/tree-ssa/pr94800.c: New test.
	* gcc.dg/tree-ssa/popcount5.c: New test.
	* gcc.dg/tree-ssa/popcount5l.c: New test.
	* gcc.dg/tree-ssa/popcount5ll.c: New test.