[InstSimplify] Don't try to constant fold AllocaInsts since it won't do anything.

Should give a small compile time improvement.

llvm-svn: 300125
diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 0e522cb..e12f640 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -4627,6 +4627,10 @@
     Result = SimplifyCastInst(I->getOpcode(), I->getOperand(0), I->getType(),
                               DL, TLI, DT, AC, I);
     break;
+  case Instruction::Alloca:
+    // No simplifications for Alloca and it can't be constant folded.
+    Result = nullptr;
+    break;
   }
 
   // In general, it is possible for computeKnownBits to determine all bits in a