Make ix86 cost of VEC_SELECT equivalent to SUBREG cost 1

Make ix86 cost of VEC_SELECT equivalent to SUBREG cost 1

This patch fixes regression of imagick with PGO and AVX512 where correcting size
cost of SSE operations (to be 4 instead of 2 originally cut&pasted from x87)
made late combine to eliminate zero registers introduced by rapd.  The problem
is that cost-model mistakely accounts VEC_SELECT as real instruction while it is
optimized to nothing if src==dest (which is the case of these testcases).
This register is used to eliminate false dependency between source and destination
of int->fp conversions.

While ix86_insn_cost hook already contains logic to incrase cost of the zero-extend
the costs was not enough.

gcc/ChangeLog:

	PR target/119900
	* config/i386/i386.cc (ix86_can_change_mode_class): Add TODO
	comment.
	(ix86_rtx_costs): Make VEC_SELECT equivalent to SUBREG cost 1.