[ARM] Don't look through pointer types in canTailPredicateLoop()

Inspecting the pointer element type here is incompatible with
opaque pointers, and doesn't seem necessary to me. I think the
intention might have been to check the type of load/store pointer
arguments, but I believe those should get checked through their
return type or value operand anyway. I don't get any test failures
if I simply drop this.

Differential Revision: https://reviews.llvm.org/D118353