Fixup dropping REG_EQUAL note in ext-dce

Followup to r16-1613-g34e1e5e33ec3eb. remove_reg_equal_equiv_notes's
2nd argument is 'no_rescan' which we accidentally had on, tripping
an assert in combine or ira because we hadn't left things in a consistent
state.

Fix the thinko by enabling rescanning.

gcc/ChangeLog:
	PR rtl-optimization/120795

	* ext-dce.cc (ext_dce_try_optimize_insn): Enable rescan in
	remove_reg_equal_equiv_notes call.

Co-authored-by: Jeff Law <jlaw@ventanamicro.com>
(cherry picked from commit cdd678544fefc313cb1c9da0327158d3ed355f62)