Fix -freorder-blocks-and-partition glitch with Windows SEH

Fix -freorder-blocks-and-partition glitch with Windows SEH

Since GCC 8, the -freorder-blocks-and-partition pass can split a function
into hot and cold parts, thus generating 2 CIEs for a single function in
DWARF for exception purposes and doing an equivalent trick for Windows SEH.

Now the Windows system unwinder is picky when it comes to the boundary
between an active EH region and the end of the function and, therefore,
a nop may need to be added in specific cases.

gcc/
	* config/i386/winnt.c (i386_pe_seh_unwind_emit): When switching to
	the cold section, emit a nop before the directive if the previous
	active instruction can throw.