[SelectionDAG] ComputeKnownBits - minimum leading/trailing zero bits in LSHR/SHL (PR44526)

As detailed in https://blog.regehr.org/archives/1709 we don't make use of the known leading/trailing zeros for shifted values in cases where we don't know the shift amount value.

This patch adds support to SelectionDAG::ComputeKnownBits to use KnownBits::countMinTrailingZeros and countMinLeadingZeros to set the minimum guaranteed leading/trailing known zero bits.

Differential Revision: https://reviews.llvm.org/D72573