re PR tree-optimization/71631 (Wrong constant folding)

re PR tree-optimization/71631 (Wrong constant folding)

	PR tree-optimization/71631
	* tree-ssa-reassoc.c (reassociate_bb): Pass true as last argument
	to rewrite_expr_tree even if negate_result, move new_lhs var
	declaration and initialization earlier, for powi_result set afterwards
	new_lhs to lhs.  For negate_result, use new_lhs instead of tmp
	if new_lhs != lhs, and don't shadow gsi var.

	* gcc.c-torture/execute/pr71631.c: New test.

From-SVN: r237782