Fix spelling mistakes in SelectionDAG comments. NFC.

Identified by Pedro Giffuni in PR27636.

llvm-svn: 287487
diff --git a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
index cf5803b..7bc0c9d 100644
--- a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
@@ -570,7 +570,7 @@
       Ops.push_back(MachineOperand::CreateImm(StackMaps::ConstantOp));
       Ops.push_back(MachineOperand::CreateImm(0));
     } else if (auto *AI = dyn_cast<AllocaInst>(Val)) {
-      // Values coming from a stack location also require a sepcial encoding,
+      // Values coming from a stack location also require a special encoding,
       // but that is added later on by the target specific frame index
       // elimination implementation.
       auto SI = FuncInfo.StaticAllocaMap.find(AI);