[ARM] Limit v6m unrolling with multiple live outs

v6m cores only have a limited number of registers available. Unrolling
can mean we spend more on stack spills and reloads than we save from the
unrolling. This patch adds an extra heuristic to put a limit on the
unroll count for loops with multiple live out values, as measured from
the LCSSA phi nodes.

Differential Revision: https://reviews.llvm.org/D104659