[AArch64] Avoid repeated hash lookups (NFC) (#130542)

Author: kazutakahirata

llvm/lib/Target/AArch64/AArch64FastISel.cpp

@@ -346,12 +346,10 @@ Register AArch64FastISel::fastMaterializeAlloca(const AllocaInst *AI) {
          "Alloca should always return a pointer.");
 
   // Don't handle dynamic allocas.
-  if (!FuncInfo.StaticAllocaMap.count(AI))
+  auto SI = FuncInfo.StaticAllocaMap.find(AI);
+  if (SI == FuncInfo.StaticAllocaMap.end())
     return Register();
 
-  DenseMap<const AllocaInst *, int>::iterator SI =
-      FuncInfo.StaticAllocaMap.find(AI);
-
   if (SI != FuncInfo.StaticAllocaMap.end()) {
     Register ResultReg = createResultReg(&AArch64::GPR64spRegClass);
     BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, MIMD, TII.get(AArch64::ADDXri),