[ARM][HWLoops] Create hardware loops for sibling loops

Given a loop with two subloops, it should be possible for both to be
converted to hardware loops. That's what this patch does, simply enough.
It slightly alters the loop iterating order to try and convert all
subloops. If one (or more) succeeds, it stops as before.

Differential Revision: https://reviews.llvm.org/D78502