Fix regression from x86 multi-epilogue tuning

Fix regression from x86 multi-epilogue tuning

With the avx512_two_epilogues tuning enabled for zen4 and zen5
the gcc.target/i386/vect-epilogues-5.c testcase below regresses
and ends up using AVX2 sized vectors for the masked epilogue
rather than AVX512 sized vectors.  The following patch rectifies
this and adds coverage for the intended behavior.

	* config/i386/i386.cc (ix86_vector_costs::finish_cost):
	Do not suggest a first epilogue mode for AVX512 sized
	main loops with X86_TUNE_AVX512_TWO_EPILOGUES as that
	interferes with using a masked epilogue.

	* gcc.target/i386/vect-epilogues-1.c: New testcase.
	* gcc.target/i386/vect-epilogues-2.c: Likewise.
	* gcc.target/i386/vect-epilogues-3.c: Likewise.
	* gcc.target/i386/vect-epilogues-4.c: Likewise.
	* gcc.target/i386/vect-epilogues-5.c: Likewise.