Bug was introduced in: https://reviews.llvm.org/D157807
The prior logic assumed that the information from the knownbits
returned from analyzing the `icmp` and its operands in the context
basicblock would be consistent.
This is not necessarily the case if we are analyzing deadcode.
For example with `(icmp sgt (select cond, 0, 1), -1)`. If we take
knownbits for the `select` using knownbits from the operator, we will
know the signbit is zero. If we are analyzing a not-taken from based
on the `icmp` (deadcode), we will also "know" that the `select` must
be negative (signbit is one). This will result in a conflict in
knownbits.
The fix is to just give up on analying the phi-node if its deadcode. We 1) don't want to waste time continuing to analyze it and 2) will be removing it (and its dependencies) later.
Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D158960
dd48a9b056