coalesce SSA versions in var-tracking notes

[Paolo Bonzini]

> 	* var-tracking.c (enum micro_operation_type): Add MO_COPY.
> 	(var_debug_decl): New function.
> 	(var_reg_set): Follow debug decl link.  Add location even if
> 	reg is already known to hold some other variable.
> 	(var_mem_set): Follow debug decl link.
> 	(var_reg_delete_and_set, var_mem_delete_and_set): Follow debug
> 	decl link.  Delete other known locations of the variable part
> 	if requested.
> 	(var_reg_delete, var_mem_delete): Delete other known locations
> 	of the variable part if requested.
> 	(same_variable_part_p): New function.
> 	(add_stores): Select MO_COPY when appropriate.
> 	(vt_initialize): Handle it.
> 	(compute_bb_dataflow, emit_notes_in_bb): Likewise.  Delete
> 	known locations for MO_SET and MO_CLOBBER.
> 	(clobber_variable_part): New function.
> 	* dwarf2out.c (dwarf2out_var_location): Do not follow debug
> 	decl link.

ChangeLog not updated.

Paolo