Use aarch64_decode_insn in aarch64_analyze_prologue

Use aarch64_decode_insn in aarch64_analyze_prologue

This patch convert aarch64_analyze_prologue to using aarch64_decode_insn
to decode instructions.  After this change, aarch64_analyze_prologue
looks much simple, and some aarch64_decode_* functions are removed
accordingly.

gdb:

2015-11-05  Yao Qi  <yao.qi@linaro.org>

	* aarch64-tdep.c (extract_signed_bitfield): Remove.
	(decode_masked_match): Remove.
	(aarch64_decode_add_sub_imm): Remove.
	(aarch64_decode_br): Remove.
	(aarch64_decode_eret): Remove.
	(aarch64_decode_movz): Remove.
	(aarch64_decode_orr_shifted_register_x): Remove.
	(aarch64_decode_ret): Remove.
	(aarch64_decode_stp_offset): Remove.
	(aarch64_decode_stur): Remove.
	(aarch64_analyze_prologue): Call aarch64_decode_insn
	and use aarch64_inst to decode instructions.