[ARM GlobalISel] Fix G_(UN)MERGE_VALUES handling after r319524

r319524 has made more G_MERGE_VALUES/G_UNMERGE_VALUES pairs legal than
are supported by the rest of the pipeline. Restrict that to only the
cases that we can currently handle: packing 32-bit values into 64-bit
ones, when we have hardware FP.

llvm-svn: 320980