[AArch64] Expand v8i8 cttz (PR39729)

Fix for https://bugs.llvm.org/show_bug.cgi?id=39729.

Rather than adding just a case for v8i8 I'm setting cttz to expand
for all vector types.

Differential Revision: https://reviews.llvm.org/D58008

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353872 91177308-0d34-0410-b5e6-96231b3b80d8