[AArch64][FastISel] Always use AND before checking the branch flag.
When we are not emitting the condition for the branch, because the condition is
in another BB or SDAG did the selection for us, then we have to mask the flag in
the register with AND.
This is required when the condition comes from a truncate, because SDAG only
truncates down to a legal size of i32.
This fixes rdar://problem/22161062.