[VectorCombine] scalarizeLoadExtract - don't create scalar loads is any extract is waiting to be erased

If any extract is waiting to be erased, then bail out as this will distort the cost calculation and possibly lead to infinite loops.

Fixes #129373

Author: RKSimon

llvm/lib/Transforms/Vectorize/VectorCombine.cpp

@@ -1579,6 +1579,11 @@ bool VectorCombine::scalarizeLoadExtract(Instruction &I) {
     if (!UI || UI->getParent() != LI->getParent())
       return false;
 
+    // If any extract is waiting to be erased, then bail out as this will
+    // distort the cost calculation and possibly lead to infinite loops.
+    if (isInstructionTriviallyDead(UI))
+      return false;
+
     // Check if any instruction between the load and the extract may modify
     // memory.
     if (LastCheckedInst->comesBefore(UI)) {

llvm/test/Transforms/VectorCombine/X86/load-extractelement-scalarization.ll

@@ -24,3 +24,15 @@ define void @multiple_extract(ptr %p) {
   store i32 %e1, ptr %p1, align 4
   ret void
 }
+
+; infinite loop if we fold an extract that is waiting to be erased
+define void @unsued_extract(ptr %p) {
+; CHECK-LABEL: @unsued_extract(
+; CHECK-NEXT:    ret void
+;
+  %load = load <4 x float>, ptr %p, align 8
+  %shuffle0 = shufflevector <4 x float> zeroinitializer, <4 x float> %load, <4 x i32> <i32 0, i32 4, i32 1, i32 5>
+  %shuffle1 = shufflevector <4 x float> %shuffle0, <4 x float> zeroinitializer, <4 x i32> <i32 0, i32 4, i32 poison, i32 poison>
+  %extract = extractelement <4 x float> %load, i64 1
+  ret void
+}