InstCombine: Remove dead code

[arsenm]

Summary

This is checking for a situation which I don't think can happen.
Before this the alignment is already explicitly set, and
non-0 sized allocas are sorted before, so any sized alloca
seen should have already been visited to set the alignment.

[arsenm]

ping

[arsenm]

ping

[majnemer]

This transform would not be correct if InstCombine removed all the allocas and then started introducing some back. We should make transforms resilient to iteration order.

lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp

if (DL.getTypeAllocSize(AI.getAllocatedType()) == 0) {
         // dominance as the array size was forced to a constant earlier already.
         AllocaInst *EntryAI = dyn_cast<AllocaInst>(FirstInst);
         if (!EntryAI || !EntryAI->getAllocatedType()->isSized() ||
             DL.getTypeAllocSize(EntryAI->getAllocatedType()) != 0) {
           AI.moveBefore(FirstInst);
           return &AI;
         }
 
-        // If the alignment of the entry block alloca is 0 (unspecified),
-        // assign it the preferred alignment.
-        if (EntryAI->getAlignment() == 0)
-          EntryAI->setAlignment(
-              DL.getPrefTypeAlignment(EntryAI->getAllocatedType()));
+        assert(EntryAI->getAlignment() != 0 &&
+               "alignment should already have been set if missing");
+
         // Replace this zero-sized alloca with the one at the start of the entry
         // block after ensuring that the address will be aligned enough for both
         // types.
         unsigned MaxAlign = std::max(EntryAI->getAlignment(),
                                      AI.getAlignment());
         EntryAI->setAlignment(MaxAlign);
         if (AI.getType() != EntryAI->getType())
           return new BitCastInst(EntryAI, AI.getType());