[arm] compute framereg and framesize when needed

[arm] compute framereg and framesize when needed

I find local variables framereg and framesize is only used when cache
isn't NULL.  This patch to move the code into "if (cache)" block.

gdb:

2014-11-29  Yao Qi  <yao@codesourcery.com>

	* arm-tdep.c (arm_analyze_prologue): Move local variables
	'framereg' and 'framesize' to inner block.  Move code to
	inner block too.