[llvm] [ValueTracking] Handle assume(!(trunc X)) in computeKnownBitsFromContext (PR #219854)

[Andreas Jonson via llvm-commits]

================
@@ -1145,6 +1145,21 @@ void llvm::computeKnownBitsFromContext(const Value *V, KnownBits &Known,
       Known.One.setBit(0);
       return;
     }
+    // assume(!(trunc V)) -> last bit of V is 0
+    Value *NotOp;
+    if (match(Arg, m_Not(m_Value(NotOp))) && Arg->hasOneUse() &&
+        NotOp->hasOneUse()) {
+      auto *NotTrunc = dyn_cast<TruncInst>(NotOp);
+      if (NotTrunc && NotTrunc->getOperand(0) == V &&
+          isValidAssumeForContext(I, Q)) {
----------------
andjo403 wrote:

maybe instead of doing the oneUse checks that is a bit of a hack we can do the ephemeral checks here as we know all possible ephemeral values something like
```suggestion
    if (match(Arg, m_Not(m_Value(NotOp, m_Trunc(m_Specific(V))))) &&
        (Q.AllowEphemerals ||
         (Q.CxtI != Arg && Q.CxtI != NotOp && Q.CxtI != V)) &&
        isValidAssumeForContext(I, Q.allowEphemerals(true))) {
      auto *NotTrunc = cast<TruncInst>(NotOp);
```

https://github.com/llvm/llvm-project/pull/219854