Canonicalize constant multiplies in division

Canonicalize constant multiplies in division

This patch implements some of the optimizations discussed in
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71026.

Canonicalize x / (C1 * y) into (x * C2) / y.

This moves constant multiplies out of the RHS of a division in order
to allow further simplifications (such as (C1 * x) / (C2 * y) ->
(C3 * x) / y) and to enable more reciprocal CSEs.

2017-11-16  Wilco Dijkstra  <wdijkstr@arm.com>
	    Jackson Woodruff  <jackson.woodruff@arm.com>

    gcc/
	PR tree-optimization/71026
	* match.pd: Canonicalize constant multiplies in division.

    gcc/testsuite/
	PR tree-optimization/71026
	* gcc.dg/cse_recip.c: New test.

Co-Authored-By: Jackson Woodruff <jackson.woodruff@arm.com>
From-SVN: r254816