[LiveIntervals] Repair subreg ranges in processTiedPairs

In TwoAddressInstructionPass::processTiedPairs, update subranges of the
live interval for RegB as well as the main range.

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.

Differential Revision: https://reviews.llvm.org/D110526