PPCDAGToDAGISel::isRunOfOnes should return false on zero

This fixes a bug (found by csmith) at -O0 where we attempt to create a RLWIMI
with an out-of-range operand. Most uses of the isRunOfOnes function are guarded
by a condition that the value is not zero. This was not true in two places, and
in both places a zero input would result in an out-of-rage MB value (= 32).

To fix this, isRunOfOnes returns false on a zero input (and I've remove one
now-redundant guard).

llvm-svn: 186101