[llvm-mca] Pass the InstrBuilder to the constructor of Backend.

This is done in preparation for the fix for PR36784.
No functional change.

llvm-svn: 328306
diff --git a/llvm/tools/llvm-mca/Backend.cpp b/llvm/tools/llvm-mca/Backend.cpp
index 94edf87..d0afa45 100644
--- a/llvm/tools/llvm-mca/Backend.cpp
+++ b/llvm/tools/llvm-mca/Backend.cpp
@@ -34,7 +34,7 @@
   while (SM.hasNext()) {
     InstRef IR = SM.peekNext();
     std::unique_ptr<Instruction> NewIS =
-        IB->createInstruction(IR.first, *IR.second);
+        IB.createInstruction(IR.first, *IR.second);
     const InstrDesc &Desc = NewIS->getDesc();
     if (!DU->isAvailable(Desc.NumMicroOps) ||
         !DU->canDispatch(IR.first, *NewIS))