gas: Refactor SFrame CFI opcode DW_CFA_register processing

Refactor SFrame processing of CFI opcode DW_CFA_register into a separate
function. This harmonizes the CFI opcode processing.

While at it reword the comment on CFI opcodes that are not processed.

This is a purely mechanical change.

gas/
	* gen-sframe.c (sframe_do_cfi_insn, sframe_xlate_do_register):
	Refactor SFrame CFI opcode DW_CFA_register processing.

Signed-off-by: Jens Remus <jremus@linux.ibm.com>