[AArch64][FastISel] Always use an AND instruction when truncating to non-legal types.

When truncating to non-legal types (such as i16, i8 and i1) always use an AND
instruction to mask out the upper bits. This was only done when the source type
was an i64, but not when the source type was an i32.

This commit fixes this and adds the missing i32 truncate tests.

This fixes rdar://problem/21990703.

llvm-svn: 243198