tree-optimization/116585 - SSA corruption with split_constant_offset

tree-optimization/116585 - SSA corruption with split_constant_offset

split_constant_offset when looking through SSA defs can end up
picking SSA leafs that are subject to abnormal coalescing.  This
can lead to downstream consumers to insert code based on the
result (like from dataref analysis) in places that violate constraints
for abnormal coalescing.  It's best to not expand defs whose operands
are subject to abnormal coalescing - and not either do something when
a subexpression has operands like that already.

	PR tree-optimization/116585
	* tree-data-ref.cc (split_constant_offset_1): When either
	operand is subject to abnormal coalescing do no further
	processing.

	* gcc.dg/torture/pr116585.c: New testcase.