[ARM] Disable WLSTP loops

This checks to see if the loop will likely become a tail predicated loop
and disables wls loop generation if so, as the likelihood for reverting
is currently too high. These should be fairly rare situations anyway due
to the way iterations and element counts are used during lowering. Just
not trying can alter how SCEV's are materialized however, leading to
different codegen.

It also adds a option to disable all while low overhead loops, for
debugging.

Differential Revision: https://reviews.llvm.org/D91663