[InstCombine] Fix assertion failure in issue80597 (#80614)

The assertion in #80597 failed when we were trying to compute known bits
of a value in an unreachable BB.

https://github.com/llvm/llvm-project/blob/859b09da08c2a47026ba0a7d2f21b7dca705864d/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp#L749-L810

In this case, `SignBits` is 30 (deduced from instr info), but `Known` is
`10000101010111010011110101000?0?00000000000000000000000000000000`
(deduced from dom cond). Setting high bits of `lshr Known, 1` will lead
to conflict.

This patch masks out high bits of `Known.Zero` to address this problem.

Fixes #80597.