[GlobalMerge] Set the alignment on merged global structs

[GlobalMerge] Set the alignment on merged global structs

If no alignment is set, the abi/preferred alignment of structs will be
used which may be higher than required. This can lead to extra padding
and in the end an increase in data size.

Differential Revision: https://reviews.llvm.org/D47633

llvm-svn: 334099