[llvm] MTM: fix style after cursory reading (NFC) (PR #100404)

[Ramkumar Ramachandra via llvm-commits]

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@@ -852,14 +846,14 @@ computeInstrDepths(const MachineBasicBlock *MBB) {
   // implies Head->HasValidInstrDepths, so we only need to start from the first
   // block in the trace that needs to be recomputed.
   SmallVector<const MachineBasicBlock*, 8> Stack;
-  do {
+  while (MBB) {
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artagnon wrote:

The MBB->getNumber() in the loop would fail on MBB = nullptr.

https://github.com/llvm/llvm-project/pull/100404