read-rtl.c: split out read_rtx_operand from read_rtx_code

gcc/ChangeLog:
	* read-rtl.c (read_rtx_code): Rename local "i" to "idx", and use
	"c" instead when parsing characters.  Move operand parsing into...
	(read_rtx_operand): ...this new function, renaming "i" to "idx",
	and tightening the scope of various locals.

From-SVN: r240502