Factor out division by squares

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

The division reciprocal optimization now handles divisions by squares:

     x / (y * y) -> x  * (1 / y) * (1 / y)

This requires at least one more division by y before it triggers - the
3 divisions of (1/ y) are then CSEd into a single division.  Overall
this changes 1 division into 1 multiply, which is generally much faster.


2017-11-24  Jackson Woodruff  <jackson.woodruff@arm.com>

    gcc/
	PR tree-optimization/71026
	* tree-ssa-math-opts (is_division_by_square, is_square_of): New.
	(insert_reciprocals): Change to insert reciprocals before a division
	by a square and to insert the square of a reciprocal.
	(execute_cse_reciprocals_1): Change to consider division by a square.
	(register_division_in): Add importance parameter.

    testsuite/
	PR tree-optimization/71026
	* gfortran.dg/extract_recip_1.f: New test.
	* gcc.dg/extract_recip_3.c: New test.
	* gcc.dg/extract_recip_4.c: New test.

From-SVN: r255141