[ARM] Another f16 litpool fix

We were always setting the block alignment to 2 bytes in Thumb mode
and 4-bytes in ARM mode (r325754, and r325012), but this could cause 
reducing the block alignment when it already had been aligned (e.g. 
in Thumb mode when the block is a CPE that was already 4-byte aligned).

Patch by Momchil Velikov, I've only added a test.

Differential Revision: https://reviews.llvm.org/D43777

llvm-svn: 326232