CodeGen: Use handy new-fangled post-increment, NFC

Drive-by cleanup; I noticed this when reviewing the patch that became
r225466.

llvm-svn: 225468
diff --git a/llvm/lib/CodeGen/CodeGenPrepare.cpp b/llvm/lib/CodeGen/CodeGenPrepare.cpp
index 7550df2..daab43e 100644
--- a/llvm/lib/CodeGen/CodeGenPrepare.cpp
+++ b/llvm/lib/CodeGen/CodeGenPrepare.cpp
@@ -4330,7 +4330,7 @@
   for (BasicBlock &BB : F) {
     Instruction *PrevNonDbgInst = nullptr;
     for (BasicBlock::iterator BI = BB.begin(), BE = BB.end(); BI != BE;) {
-      Instruction *Insn = BI; ++BI;
+      Instruction *Insn = BI++;
       DbgValueInst *DVI = dyn_cast<DbgValueInst>(Insn);
       // Leave dbg.values that refer to an alloca alone. These
       // instrinsics describe the address of a variable (= the alloca)