tree-optimization/105263 - reassoc and DFP

tree-optimization/105263 - reassoc and DFP

reassoc has certain tricks which in the end depend on the ability
to undo them.  For DFP creating a -1. constant is easy but
re-identifying is appearantly not - real_minus_onep rejects those
outright for DFP.  So we have to disable (at least) this one trick.

2022-04-13  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/105263
	* tree-ssa-reassoc.cc (try_special_add_to_ops): Do not consume
	negates in multiplication chains with DFP.

	* gcc.dg/pr105263.c: New testcase.