objtool,x86: Teach decode about LOOP* instructions

When 'discussing' control flow Masami mentioned the LOOP* instructions
and I realized objtool doesn't decode them properly.

As it turns out, these instructions are somewhat inefficient and as
such unlikely to be emitted by the compiler (a few vmlinux.o checks
can't find a single one) so this isn't critical, but still, best to
decode them properly.

Reported-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/Yxhd4EMKyoFoH9y4@hirez.programming.kicks-ass.net

tools/objtool/arch/x86/decode.c

@@ -635,6 +635,12 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
 		*type = INSN_CONTEXT_SWITCH;
 		break;
 
+	case 0xe0: /* loopne */
+	case 0xe1: /* loope */
+	case 0xe2: /* loop */
+		*type = INSN_JUMP_CONDITIONAL;
+		break;
+
 	case 0xe8:
 		*type = INSN_CALL;
 		/*