[RegisterCoalescer] Fix updating LiveIntervals in joinReservedPhysReg

Live intervals for physical registers are calculated lazily on demand.
In a case like this:

  16B    %0:gpr32 = IMPLICIT_DEF
  32B    $wzr = COPY %0

if the live interval for $wzr did not already exist then the update code
in joinReservedPhysReg would create it with a definition at 32B, which
would remain even after the COPY was deleted.

Differential Revision: https://reviews.llvm.org/D151314