tree-optimization/115764 - testcase for BB SLP issue

The following adds a testcase for a CSE issue with BB SLP two operator
handling when we make those CSE aware by providing SLP_TREE_SCALAR_STMTS
for them.  This was reduced from 526.blender_r.

	PR tree-optimization/115764
	* gcc.dg/vect/bb-slp-76.c: New testcase.