Don't splice in vector creation when optimizing vector-length.

Closes PR13788.

original commit: d9264525bd823052cd456df0caa900eac2d87ed3

collects/tests/typed-racket/optimizer/tests/pr13788.rkt

@@ -0,0 +1,9 @@
+#;
+(
+TR opt: pr13788.rkt 9:0 (vector-length (vector 1 2 3)) -- known-length vector-length
+3
+)
+
+#lang typed/racket
+
+(vector-length (vector 1 2 3)) ; should not print the vector

collects/typed-racket/optimizer/vector.rkt

@@ -44,7 +44,7 @@
                   (add-disappeared-use #'op)
                   (match (type-of #'v)
                     [(tc-result1: (HeterogeneousVector: es))
-                     #`(begin v.opt #,(length es))]))) ; v may have side effects
+                     #`(let () v.opt #,(length es))]))) ; v may have side effects
   ;; we can optimize vector-length on all vectors.
   ;; since the program typechecked, we know the arg is a vector.
   ;; we can optimize no matter what.