Use auto_vec in ssa_equiv_stack.

Use auto_vec in ssa_equiv_stack.

There is a mismatch between the new and the delete for the
ssa_equiv_stack class.  The correct idiom should have been delete[].
It has been pointed out that perhaps a better alternative is to use
an auto_vec which does everything automatically.  Plus, it is more
consistent with m_stack which is already an auto_vec.

This patch fixes the issue in PR100984.

Tested on x86-64 Linux.

gcc/ChangeLog:

        PR tree-optimization/100984
	* gimple-ssa-evrp.c  (ssa_equiv_stack): Use auto_vec for
	replacements table.
	(ssa_equiv_stack::~ssa_equiv_stack): Remove.