rtl-ssa: Create REG_UNUSED notes after all pending changes

rtl-ssa: Create REG_UNUSED notes after all pending changes

Unlike REG_DEAD notes, REG_UNUSED notes need to be kept free of
false positives by all passes.  function_info::change_insns
does this by removing all REG_UNUSED notes, and then using
add_reg_unused_notes to add notes back (or create new ones)
where appropriate.

The problem was that it called add_reg_unused_notes on the fly
while updating each instruction, which meant that the information
for later instructions in the change set wasn't up to date.
This patch does it in a separate loop instead.

gcc/
	* rtl-ssa/changes.cc (function_info::apply_changes_to_insn): Remove
	call to add_reg_unused_notes and instead...
	(function_info::change_insns): ...use a separate loop here.