Fix amd64 windows unwinding issues within MS dlls.

Fix amd64 windows unwinding issues within MS dlls.

Unwind info in system dlls uses almost all possible codes, contrary to unwind
info generated by gcc.  A few issues have been discovered: incorrect handling
of SAVE_NONVOL opcodes and incorrect in prologue range checks.  Furthermore I
added comments not to forget what has been investigated.

gdb/ChangeLog:
	* amd64-windows-tdep.c (amd64_windows_find_unwind_info): Move
	redirection code to ...
	(amd64_windows_frame_decode_insns): ... Here.  Fix in prologue
	checks.  Fix SAVE_NONVOL operations.  Add debug code and comments.