ARM: Fix cttz expansion on vector types.

The 64/128-bit vector types are legal if NEON instructions are
available.  However, there was no matching patterns for @llvm.cttz.*()
intrinsics and result in fatal error.

This commit fixes the problem by lowering cttz to:
a. ctpop((x & -x) - 1)
b. width - ctlz(x & -x) - 1

llvm-svn: 242037