Re: PR27723, Internal error in select_cie_for_fde

Let's make sure what we allow in the CIE initial instructions and what
select_cie_for_fde compares for a match is always in sync.  Also
correct the previous patch that allowed DW_CFA_GNU_window_save to be
part of the CIE initial instructions, which was likely a mistake.

	PR 27723
	* dw2gencfi.c (initial_cie_insn): New function, extracted from..
	(select_cie_for_fde): ..here.  Simplify.