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.

gcc/config/i386/i386.md

@@ -620,13 +620,11 @@
 	(const_int 0)))
 ;; There are also additional prefixes in 3DNOW, SSSE3.
-;; ssemuladd,sse4arg default to 0f24/0f25 and DREX byte,
-;; sseiadd1,ssecvt1 to 0f7a with no DREX byte.
 ;; 3DNOW has 0f0f prefix, SSSE3 and SSE4_{1,2} 0f38/0f3a.
+;; While generally inapplicable to VEX/XOP/EVEX encodings, "length_vex" uses
+;; the attribute evaluating to zero to know that VEX2 encoding may be usable.
 (define_attr "prefix_extra" ""
-  (cond [(eq_attr "type" "ssemuladd,sse4arg")
-	   (const_int 2)
-	 (eq_attr "type" "sseiadd1,ssecvt1")
+  (cond [(eq_attr "type" "ssemuladd,sse4arg,sseiadd1,ssecvt1")
 	   (const_int 1)
 	]
 	(const_int 0)))