[LAA] Check independence of strided accesses before forward case

This patch changes the order in which we attempt to prove the independence of
strided accesses. We previously did this after we knew the dependence distance
was positive. With this change, we check for independence before handling the
negative distance case. The patch prevents LAA from reporting forward
dependences for independent strided accesses.

This change was requested in the review of D19984.

llvm-svn: 270072