x86: "prefix_extra" tidying

x86: "prefix_extra" tidying

Drop SSE5 leftovers from both its comment and its default calculation.
A value of 2 simply cannot occur anymore. Instead extend the comment to
mention the use of the attribute in "length_vex", clarifying why
"prefix_extra" can actually be meaningful on VEX-encoded insns despite
those not having any real prefixes except possibly segment overrides.

gcc/

	* config/i386/i386.md (prefix_extra): Correct comment. Fold
	cases yielding 2 into ones yielding 1.