regrename: Use PC instead of CC0 to hide operands

regrename: Use PC instead of CC0 to hide operands

The regrename pass temporarily changes some operand RTL to CC0 so that
note_stores and scan_rtx don't see those operands.  CC0 is deprecated
and we want to remove it, so we need to use something else here.
PC fits the bill fine.

	* regrename.c (hide_operands): Use pc_rtx instead of cc0_rtx.
	(build_def_use): Use PC instead of CC0 in a comment.

From-SVN: r276401