Sanjay Patel a954b8a363 [ValueTracking] fix CannotBeNegativeZero() to disregard 'nsz' FMF ...

The 'nsz' flag is different than 'nnan' or 'ninf' in that it does not create poison.
Make that explicit in the LangRef and fix ValueTracking analysis that misinterpreted
the definition.

This manifests as bugs in InstSimplify shown in the test diffs and as discussed in
PR45778:
https://bugs.llvm.org/show_bug.cgi?id=45778

Differential Revision: https://reviews.llvm.org/D79422

2020-05-05 16:04:59 -04:00

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