[InstCombine] Prevent the transform of the comparison for all undef elements

[Allen]

Summary

Some pass such as Loop Vectorization(-fvectorize) may generate undef elements
constant, which will ball out the combine optimization.

[lebedev.ri]

The test already passes without the code change.

[Allen]

In D107691#2932588, @lebedev.ri wrote:

The test already passes without the code change.

Oh, sorry. this bug fix with 2813acb7d1 Update m_Undef to match vectors/aggrs with undefs and poisons mixed, my local code is too old.

[Allen]

Abandon as this bug already fixed with the newest commit

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

     return llvm::None;
   }
 
+  // May be all elements are under value
+  if (C->containsUndefOrPoisonElement() && SafeReplacementConstant == nullptr)
+    return llvm::None;
+
   // It may not be safe to change a compare predicate in the presence of
   // undefined elements, so replace those elements with the first safe constant
   // that we found.
   // TODO: in case of poison, it is safe; let's replace undefs only.
   if (C->containsUndefOrPoisonElement()) {
-    assert(SafeReplacementConstant && "Replacement constant not set");
     C = Constant::replaceUndefsWith(C, SafeReplacementConstant);
   }
 

llvm/test/Transforms/InstCombine/icmp-bc-vec-inseltpoison.ll

   %cmp = icmp slt i27 %cast, 262657 ; 0x040201
   ret i1 %cmp
 }
+
+; Verify the case that all elements are under define
+define <4 x i1> @test_i32_ule_pattern(i32 %val) {
+; CHECK-LABEL: @test_i32_ule_pattern(
+; CHECK-NEXT:    ret <4 x i1> <i1 true, i1 true, i1 true, i1 true>
+;
+  %insvec = insertelement <4 x i32> poison, i32 %val, i32 0
+  %vec = shufflevector <4 x i32> %insvec, <4 x i32> poison, <4 x i32> zeroinitializer
+  %cond = icmp ule <4 x i32> %vec, <i32 undef, i32 poison, i32 poison, i32 poison>
+  ret <4 x i1> %cond
+}