Struct byval: use the correct alignment for loads generated to load
from struct byval to registers.

We used to pass 0 which means the alignment of PtrVT. Even when the alignment
of the struct is smaller than 4, the LOADs would have alignment of 4, and
further optimizations could combine the LOADs into a ldm, which would
cause crash.

The fix is to pass the alignment of the struct byval.

rdar://problem/15144402