Generic: Make isMask_N and isShiftedMask_N consistent over 0

Previously, isMask_N returned false for 0 but isShiftedMask_N returned true.
Almost all uses are for pattern matching bitfield operations in the backends,
and expect false (this was discovered because of AArch64's copy of this logic).

Unfortunately, I couldn't put together a small non-fragile test for this. The
nature of the bitfield operations means that this edge case is only really
triggered for nodes like "(and N, 0)", which the DAG combiner is usually very
good at folding away before they get to this stage.

rdar://20501377

llvm-svn: 234659