rs6000: allow cunroll to grow size according to -funroll-loop or -fpeel-loops
Previously, flag_unroll_loops was turned on at -O2 implicitly. This
also turned on cunroll with allowance size increasing, and cunroll
will unroll/peel the loop even the loop is complex like code in PR95018.
With this patch, size growth for cunroll is allowed only for if -funroll-loops
or -fpeel-loops or -O3 is specified explicitly.