Fix comment in rtl.def

Commit r210360 removed the first "i" field from the various instruction
nodes in rtx.def, moving it to an explicit "int insn_uid;" field
of the union "u2" within rtx_def.

Update the comment in rtl.def to reflect this change.  Also, fix
a stray apostrophe.

gcc/ChangeLog:
	* rtl.def: Update comment for "things in the instruction chain" to
	reflect the removal of the leading "i" field for INSN_UID in
	r210360.  Fix bogus apostrophe.

From-SVN: r235520