If the old instructions mentioned a subreg that the new instructions do
not, remove the subrange for that subreg.
For example, in TwoAddressInstructionPass::eliminateRegSequence, if a
use operand in the REG_SEQUENCE has the undef flag then we don't
generate a copy for it so after the elimination there should be no live
interval at all for the corresponding subreg of the def.
This is a small step towards switching TwoAddressInstructionPass over
from LiveVariables to LiveIntervals. Currently this path is only tested
if you explicitly enable -early-live-intervals.