tree-optimization/96370 - make reassoc expr rewrite more robust

In the face of the more complex tricks in reassoc with respect
to negate processing it can happen that the expression rewrite
is fooled to recurse on a leaf and pick up a bogus expression
code.  The following patch makes the expression rewrite more
robust in providing the expression code to it directly since
it is the same for all operations in a chain.

2020-07-30  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/96370
	* tree-ssa-reassoc.c (rewrite_expr_tree): Add operation
	code parameter and use it instead of picking it up from
	the stmt that is being rewritten.
	(reassociate_bb): Pass down the operation code.

	* gcc.dg/pr96370.c: New testcase.