[AArch64] Only apply bool vector bitcast opt if result is scalar (#81256)

This optimization tries to optimize bitcasts from `<N x i1>` to iN, but
currently also triggers for `<N x i1>` to `<M x iK>` bitcasts, if custom
lowering has been requested for these for an unrelated reason. Fix this
by explicitly checking that the result type is scalar.

Fixes https://github.com/llvm/llvm-project/issues/81216.