Do not track subregister liveness when it brings no benefits

Do not track subregister liveness when it brings no benefits

Some subregisters are only to indicate different access sizes, while not
providing any way to actually divide the register up into multiple
disjunct parts. Avoid tracking subregister liveness in these cases as it
is not beneficial.

Differential Revision: http://reviews.llvm.org/D8429

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232695 91177308-0d34-0410-b5e6-96231b3b80d8