rs6000: re-enable web and rnreg with -funroll-loops
Previously, limited unrolling was enabled at O2 for powerpc in r278034. At that
time, -fweb and -frename-registers were not enabled together with -funroll-loops
even for -O3. After that, we notice there are some performance degradations on
SPEC2006fp which caused by without web and rnreg. This patch enable -fweb
and -frename-registers for -funroll-loops to align original behavior before
r278034.