[SelectionDAG] Drop unnecessary lower bound check in `lowerRangeToAssertZExt` (#196785)
Drop the `Lo.isMinValue()` check in `lowerRangeToAssertZExt`.
The check was introduced in 2bba779272a2 when the bit width was computed
via `logBase2(Hi)`, which required `Lo == 0` for correctness. It is no
longer needed since 9e04befb0979 when we switched to
`getUnsignedMax().getActiveBits()` for bit width.
The change in `DAGCombiner.cpp` is to prevent a regression in
`llvm/test/CodeGen/AMDGPU/llvm.amdgcn.wavefrontsize.ll`. I wasn't able
to construct an individual test for it.
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Co-authored-by: nikic <github@npopov.com>
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