tree-optimization/126965 - limit recursive PRE expression generation

As I've recently figured the PRE value graph can have cycles so
when recursively generating a complex expression that should be
prone to run into those cycles as well.  PR126965 now has a testcase
for that, so limit recursinve expression generation.

	PR tree-optimization/126965
	* tree-ssa-pre.cc (find_or_generate_expression): Limit
	recursion depth.

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