[Hexagon] Rerun bit tracker on new instructions in RIE

Consider this case:
  vreg1 = A2_zxth vreg0   (1)
  ...
  vreg2 = A2_zxth vreg1   (2)

Redundant instruction elimination could delete the instruction (1)
because the user (2) only cares about the low 16 bits. Then it could
delete (2) because the input is already zero-extended. The problem
is that the properties allowing each individual instruction to be
deleted depend on the existence of the other instruction, so either
one can be deleted, but not both.
The existing check for this situation in RIE was insufficient. The
fix is to update all dependent cells when an instruction is removed
(replaced via COPY) in RIE.

llvm-svn: 276792