fix(amd64): correct guard displacements, frameless FP offsets and immediate ranges

Assisted-by: GLM 5.3

asm/encodable.go

@@ -36,10 +36,15 @@ func Encodable(mnemonic string) bool {
 	}
 
 	// CMOV carries size then condition (CMOVLGT); SET carries the condition
-	// alone (SETNE).
+	// alone (SETNE).  The size letter is checked exactly as encodeCmov does,
+	// so a spelling like CMOVBGT is not reported encodable when Encode
+	// would reject it.
 	if rest, ok := strings.CutPrefix(upper, "CMOV"); ok && len(rest) >= 2 {
-		if _, ok := jccMap[rest[1:]]; ok {
-			return true
+		switch rest[0] {
+		case 'W', 'L', 'Q':
+			if _, ok := jccMap[rest[1:]]; ok {
+				return true
+			}
 		}
 	}
 	if rest, ok := strings.CutPrefix(upper, "SET"); ok {