[NFC][CVP] processUDivOrURem(): refactor to use ConstantRange::getActiveBits()

As an exhaustive test shows, this logic is fully identical to the old
implementation, with exception of the case where both of the operands
had empty ranges:

```
TEST_F(ConstantRangeTest, CVP_UDiv) {
  unsigned Bits = 4;
  EnumerateConstantRanges(Bits, [&](const ConstantRange &CR0) {
    if(CR0.isEmptySet())
      return;
    EnumerateConstantRanges(Bits, [&](const ConstantRange &CR1) {
      if(CR0.isEmptySet())
        return;

      unsigned MaxActiveBits = 0;
      for (const ConstantRange &CR : {CR0, CR1})
        MaxActiveBits = std::max(MaxActiveBits, CR.getActiveBits());

      ConstantRange OperandRange(Bits, /*isFullSet=*/false);
      for (const ConstantRange &CR : {CR0, CR1})
        OperandRange = OperandRange.unionWith(CR);
      unsigned NewWidth = OperandRange.getUnsignedMax().getActiveBits();

      EXPECT_EQ(MaxActiveBits, NewWidth) << CR0 << " " << CR1;
    });
  });
}
```