Match instruction adjusts SP in thumb

Match instruction adjusts SP in thumb

This is a refactor patch, that moves matching instructions adjusting
SP into a new function, thumb_instruction_restores_sp.  The second
call to thumb_instruction_restores_sp in thumb_in_function_epilogue_p
is a little different from the original.  The original code matches
'POP <registers> without PC', but thumb_in_function_epilogue_p matches
'POP <registers> (with and without PC)'.  However, GDB found one
instruction about return and is scanning the previous instruction,
which should be an instruction about return too, so the code change
doesn't affect the functionality.

gdb:

2014-07-11  Yao Qi  <yao@codesourcery.com>

	* arm-tdep.c (thumb_instruction_restores_sp): New function.
	(thumb_in_function_epilogue_p): Call
	thumb_instruction_restores_sp.