[LAA] Support memchecks involving loop-invariant addresses

We used to only allow SCEVAddRecExpr for pointer expressions in order to
be able to compute the bounds.  However this is also trivially possible
for loop-invariant addresses (scUnknown) since then the bounds are the
address itself.

Interestingly, we used allow this for the special case when the
loop-invariant address happens to also be an SCEVAddRecExpr (in an outer
loop).

There are a couple more loops that are vectorized in SPEC after this.
My guess is that the main reason we don't see more because for example a
loop-invariant load is vectorized into a splat vector with several
vector-inserts.  This is likely to make the vectorization unprofitable.
I.e. we don't notice that a later LICM will move all of this out of the
loop so the cost estimate should really be 0.

llvm-svn: 264243