wanted: elide vector bounds checking for index with constant offsets

[3b]

sbcl can elide bounds checks when it knows (< index (length vector)), but not more complicated cases.

When writing an unrolled loop to iterate over a vector, it would be nice if it could also skip the checks when it knows index is less than length - N for some constant N, and access is (+ index M) for some M<N

for example:
(loop for i below (- (length v) 2) by 2
  sum (aref v i)
  sum (aref v (1+ i)))

(possibly a duplicate of bug #1830313 but filed separately since this needs to track ranges rather than just a transitive relation)