tree-optimization/125786 - do not rewrite stmts in reassoc

We may not simply rewrite def stmts of associative operands as
they might have multiple uses.  The following appropriately
creates a new stmt instead.

	PR tree-optimization/125786
	* tree-ssa-reassoc.cc (optimize_vec_cond_expr): Create a new
	stmt instead of rewriting an existing possibly multi-use one.

	* gcc.dg/torture/pr125786.c: New testcase.