[TargetLowering] Use ComputeMaxSignificantBits instead of ComputeNumSignBits in expandMUL_LOHI. NFC

The way ComputeNumSignBits was being used was only correct if
OuterBitSize is exactly 2x InnerBitSize. Which is always true,
but not obviously so. Comparing ComputeMaxSignificantBits to
InnerBitSize feels more correct.