[Patch match.pd] Fold (A / (1 << B)) to (A >> B)

[Patch match.pd] Fold (A / (1 << B)) to (A >> B)

For the testcase in the patch:

  unsigned long
  f2 (unsigned long a, int b)
  {
    unsigned long x = 1UL << b;
    return a / x;
  }

We currently generate an unsigned division and a left shift, where
we could instead generate a right shift.

gcc/

2017-06-21  James Greenhalgh  <james.greenhalgh@arm.com>

	* match.pd (A / (1 << B) -> A >> B): New.
	* generic-match-head.c: Include optabs-tree.h.
	* gimple-match-head.c: Likewise.
	* optabs-tree.h (target_supports_op_p): New.
	* optabs-tree.c (target_supports_op_p): New.

gcc/testsuite/

2017-06-21  James Greenhalgh  <james.greenhalgh@arm.com>

	* gcc.dg/tree-ssa/forwprop-37.c: New.

From-SVN: r249502