[llvm] [InstCombine] Fold `(trunc X)` into `X & Mask` inside `decomposeBitTestICmp` (PR #171195)

[Andreas Jonson via llvm-commits]

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@@ -153,15 +153,26 @@ llvm::decomposeBitTestICmp(Value *LHS, Value *RHS, CmpInst::Predicate Pred,
   case ICmpInst::ICMP_NE: {
     assert(DecomposeAnd);
     const APInt *AndC;
-    Value *AndVal;
-    if (match(LHS, m_And(m_Value(AndVal), m_APIntAllowPoison(AndC)))) {
-      LHS = AndVal;
+    Value *X;
+
+    if (match(LHS, m_And(m_Value(X), m_APIntAllowPoison(AndC)))) {
+      LHS = X;
       Result.Mask = *AndC;
       Result.C = C;
       Result.Pred = Pred;
       break;
     }
 
+    // Try to convert (trunc X) eq/ne C into (X & Mask) eq/ne C
+    if (LookThroughTrunc && match(LHS, m_Trunc(m_Value(X)))) {
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andjo403 wrote:

to me it sounds a bit strange that this is guarded by DecomposeAnd as there is no and in this case but I do not know

https://github.com/llvm/llvm-project/pull/171195