[GlobalISel] Remove [un]packRegs from IRTranslator

Remove the last use of packRegs from IRTranslator and delete
pack/unpackRegs. This introduces a fallback to DAGISel for intrinsics
with aggregate arguments, since we don't have a testcase for them so
it's hard to tell how we'd want to handle them.

Discussed in https://reviews.llvm.org/D63551

llvm-svn: 364514