widening_mul: restrict ops to be defined in the same basic-block when convert plusmin...

widening_mul: restrict ops to be defined in the same basic-block when convert plusminus to widen

In the testcase for PR94269, widening_mul moves two multiply
instructions from outside the loop to inside
the loop, merging with two add instructions separately.  This
increases the cost of the loop.  Like FMA detection
in the same pass, simply restrict ops to be defined in the same
basic-block to avoid possibly moving multiply
to a different block with a higher execution frequency.

2020-03-26  Felix Yang  <felix.yang@huawei.com>

	PR tree-optimization/94269
	* tree-ssa-math-opts.c (convert_plusminus_to_widen): Restrict
	this
	operation to single basic block.

	* gcc.dg/pr94269.c: New test.