[DXIL] Avoid infinite loop / stack overflow on invalid instruction

[bob80905]

The simplify instruction pass has a function called findScalarElement.
This function will possibly recurse infinitely into the first operand of the instruction that it is passed. If this instruction, for example, is %2 = insertelement <2 x i32> %2, i32 %sub, i32 0
Then the pass will recurse into the first operand of the insertelement instruction, which is the same insertelement instruction, and so on. This PR prevents infinite recursion and stack overflow by bailing out of the function early if there is a detection of the same value in the first operand.
Fixes #7034

[github-actions]

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff a8a4e98a2367080af683c48feedd7f7481a31a96 d5a7b5f3df4ffe8289fb1d808844afe753a57147 -- lib/Analysis/VectorUtils2.cpp

View the diff from clang-format here.

diff --git a/lib/Analysis/VectorUtils2.cpp b/lib/Analysis/VectorUtils2.cpp
index b5f57e3b..97bae15a 100644
--- a/lib/Analysis/VectorUtils2.cpp
+++ b/lib/Analysis/VectorUtils2.cpp
@@ -42,7 +42,7 @@ llvm::Value *llvm::findScalarElement(llvm::Value *V, unsigned EltNo) {
     if (EltNo == IIElt)
       return III->getOperand(1);
 
-// Guard against infinite loop on malformed, unreachable IR.
+    // Guard against infinite loop on malformed, unreachable IR.
     if (III == III->getOperand(0))
       return nullptr;
 

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