[TwoAddressInstructionPass] Update existing physreg live intervals

In TwoAddressInstructionPass::processTiedPairs with
-early-live-intervals, update any preexisting physreg live intervals,
as well as virtreg live intervals. By default (without
-precompute-phys-liveness) physreg live intervals only exist for
registers that are live-in to some basic block.

Differential Revision: https://reviews.llvm.org/D113191