Replace PREV_INSN et al macros with functions

Replace PREV_INSN et al macros with functions

gcc/
2014-08-19  David Malcolm  <dmalcolm@redhat.com>

	* rtl.h (PREV_INSN): Convert to an inline function.  Strengthen
	the return type from rtx to rtx_insn *,  which will enable various
	conversions in followup patches.  For now this is is done by a
	checked cast.
	(NEXT_INSN): Likewise.
	(SET_PREV_INSN): Convert to an inline function.  This is intended
	for use as an lvalue, and so returns an rtx& to allow in-place
	modification.
	(SET_NEXT_INSN): Likewise.

From-SVN: r214152