[PowerPC] Fix lowering of byval parameters for sizes greater than 8 bytes.

To store a byval parameter the existing code would store as many 8 byte elements
as was required to store the full size of the byval parameter.
For example, a paramter of size 16 would store two element of 8 bytes.
A paramter of size 12 would also store two elements of 8 bytes.
This would sometimes store too many bytes as the size of the paramter is not
always a factor of 8.

This patch fixes that issue and now byval paramters are stored with the correct
number of bytes.

Reviewed By: nemanjai, #powerpc, quinnp, amyk

Differential Revision: https://reviews.llvm.org/D121430