[decompiler] clean up vector detection and add vector float product detection (#638)

* change

* recognize vector float product and update tests

test/decompiler/reference/kernel/gstring_REF.gc

@@ -248,8 +248,7 @@
    (set! (-> str-ptr 0) (the-as uint char))
    (set! (-> str-ptr 1) (the-as uint 0))
    )
-  (let ((v1-1 0))
-   )
+  0
   0
   )
 
@@ -405,8 +404,7 @@
      )
     (set! (-> str-ptr 0) (the-as uint 0))
     )
-   (let ((v1-3 0))
-    )
+   0
    )
   #f
   )
@@ -437,8 +435,7 @@
      )
     (set! (-> ptr 1) (the-as uint 0))
     )
-   (let ((v1-7 0))
-    )
+   0
    )
   #f
   )