Don't handle lret/iret when -mlfence-before-ret=[or|not|shl|yes] since they are inval...

Don't handle lret/iret when -mlfence-before-ret=[or|not|shl|yes] since they are invalid in SGX enclaves.

gas/ChangeLog
	* gas/config/tc-i386.c: Not handle lret/iret.
	* gas/testsuite/gas/i386/lfence-ret-a.d: Adjust testcase.
	* gas/testsuite/gas/i386/lfence-ret-b.d: Ditto.
	* gas/testsuite/gas/i386/lfence-ret-c.d: Ditto.
	* gas/testsuite/gas/i386/lfence-ret-d.d: Ditto.
	* gas/testsuite/gas/i386/lfence-ret.s: Ditto.
	* gas/testsuite/gas/i386/x86-64-lfence-ret-a.d: Ditto.
	* gas/testsuite/gas/i386/x86-64-lfence-ret-b.d: Ditto.
	* gas/testsuite/gas/i386/x86-64-lfence-ret-c.d: Ditto.
	* gas/testsuite/gas/i386/x86-64-lfence-ret-d.d: Ditto.
	* gas/testsuite/gas/i386/x86-64-lfence-ret-e.d: Ditto.
	* gas/testsuite/gas/i386/x86-64-lfence-ret.s: Ditto.
	* gas/testsuite/gas/i386/x86-64-lfence-ret.e: Deleted.