Improve -mlfence-after-load

Improve -mlfence-after-load

  1.Implict load for POP/POPF/POPA/XLATB, no load for Anysize insns
  2. Add -mlfence-before-ret=shl/yes, adjust operand size of
  or/not/shl according to ret's.
  3. Issue warning for REP CMPS/SCAS since they would affect control
  flow behavior.
  4. Adjust testcases and documents.

gas/Changelog:
	* config/tc-i386.c (lfence_before_ret_shl): New member.
	(load_insn_p): implict load for POP/POPA/POPF/XLATB, no load
	for Anysize insns.
	(insert_after_load): Issue warning for REP CMPS/SCAS.
	(insert_before_before): Handle iret, Handle
	-mlfence-before-ret=shl, Adjust operand size of or/not/shl to ret's,
	(md_parse_option): Change -mlfence-before-ret=[none|not|or] to
	-mlfence-before-ret=[none/not/or/shl/yes].
	Enable -mlfence-before-ret=shl when
	-mlfence-beofre-indirect-branch=all and no explict -mlfence-before-ret option.
	(md_show_usage): Ditto.
	* doc/c-i386.texi: Ditto.
	* testsuite/gas/i386/i386.exp: Add new testcases.
	* testsuite/gas/i386/lfence-load-b.d: New.
	* testsuite/gas/i386/lfence-load-b.e: New.
	* testsuite/gas/i386/lfence-load.d: Modified.
	* testsuite/gas/i386/lfence-load.e: New.
	* testsuite/gas/i386/lfence-load.s: Modified.
	* testsuite/gas/i386/lfence-ret-a.d: Modified.
	* testsuite/gas/i386/lfence-ret-b.d: Modified.
	* testsuite/gas/i386/lfence-ret-c.d: New.
	* testsuite/gas/i386/lfence-ret-d.d: New.
	* testsuite/gas/i386/lfence-ret.s: Modified.
	* testsuite/gas/i386/x86-64-lfence-load-b.d: New.
	* testsuite/gas/i386/x86-64-lfence-load.d: Modified.
	* testsuite/gas/i386/x86-64-lfence-load.s: Modified.
	* testsuite/gas/i386/x86-64-lfence-ret-a.d: Modified.
	* testsuite/gas/i386/x86-64-lfence-ret-b.d: Modified.
	* testsuite/gas/i386/x86-64-lfence-ret-c.d: New.
	* testsuite/gas/i386/x86-64-lfence-ret-d.d: New
	* testsuite/gas/i386/x86-64-lfence-ret-e.d: New.
	* testsuite/gas/i386/x86-64-lfence-ret.e: New.
	* testsuite/gas/i386/x86-64-lfence-ret.s: New.