[GISel]: While constructing the GISelWorklist make sure we reserve at least the required size to the underlying dense map.

https://reviews.llvm.org/D57931

This should save some unnecessary growing of the DenseMap.

llvm-svn: 353498

llvm/include/llvm/CodeGen/GlobalISel/GISelWorkList.h

@@ -33,7 +33,7 @@ class GISelWorkList {
   DenseMap<MachineInstr *, unsigned> WorklistMap;
 
 public:
-  GISelWorkList() {}
+  GISelWorkList() : WorklistMap(N) {}
 
   bool empty() const { return WorklistMap.empty(); }