[FastISel][AArch64] Fix an incorrect kill flag due to a bug in SelectTrunc.

When we select a trunc instruction we don't emit any code if the type is already
i32 or smaller. This is because the instruction that uses the truncated value
will deal with it.

This behavior can incorrectly transfer a kill flag, which was meant for the
result of the truncate, onto the source register.

%2 = trunc i32 %1 to i16
... = ... %2                -> ... = ... vreg1 <kill>
... = ... %1                   ... = ... vreg1

This commit fixes this by emitting a COPY instruction, so that the result and
source register are distinct virtual registers.

This fixes rdar://problem/18178188.