[InstCombine] Fold binop of shifts with related amounts

Fold

  binop(shift(ShiftedC1, ShAmt), shift(ShiftedC2, add(ShAmt, AddC)))
    ->
  shift(binop(ShiftedC1, shift(ShiftedC2, AddC)), ShAmt)

where both shifts are the same and AddC is a valid shift amount.

Proofs: https://alive2.llvm.org/ce/z/PhVVeg

Differential Revision: https://reviews.llvm.org/D152927