[SelectionDAG] Fix incorrect condition for shift amount truncation

Comment says:
  // If the operand is larger than the shift count type but the shift
  // count type has enough bits to represent any shift value ...

It clearly talks about the shifted operand, not the shift-amount operand,
but the comparison is performed against Log2_32_Ceil(Op2.getValueSizeInBits())
where Op2 is the shift amount operand. This comparison also doesn't make
sense in the context of the previous one (ShiftsSize > Op2Size) because
Op2Size == Op2.getValueSizeInBits(). Fix to use Op1.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D110509