[PPCGCodeGeneration] Compute element size in bytes for arrays correctly.

Previously, we used to compute this with `elementSizeInBits / 8`. This
would yield an element size of 0 when the array had element size < 8 in
bits.

To fix this, ask data layout what the size in bytes should be.

Differential Revision: https://reviews.llvm.org/D36459

llvm-svn: 310448