ARM: kprobes: check register usage for probed instruction.

This patch utilizes the previously introduced checker to check
register usage for probed ARM instruction and saves it in a mask.
A further patch will use such information to avoid simulation or
emulation.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Reviewed-by: Jon Medhurst <tixy@linaro.org>
Signed-off-by: Jon Medhurst <tixy@linaro.org>

arch/arm/probes/decode.c

@@ -435,6 +435,13 @@ probes_decode_insn(probes_opcode_t insn, struct arch_probes_insn *asi,
 	 */
 	asi->stack_space = 0;
 
+	/*
+	 * Similarly to stack_space, register_usage_flags is filled by
+	 * checkers. Its default value is set to ~0, which is 'all
+	 * registers are used', to prevent any potential optimization.
+	 */
+	asi->register_usage_flags = ~0UL;
+
 	if (emulate)
 		insn = prepare_emulated_insn(insn, asi, thumb);