[SPIRV] Avoid repeated hash lookups (NFC) (#109243)

Author: kazutakahirata

llvm/lib/Target/SPIRV/SPIRVGlobalRegistry.h

@@ -255,11 +255,7 @@ class SPIRVGlobalRegistry {
 
   // Add a record about forward function call.
   void addForwardCall(const Function *F, MachineInstr *MI) {
-    auto It = ForwardCalls.find(F);
-    if (It == ForwardCalls.end())
-      ForwardCalls[F] = {MI};
-    else
-      It->second.insert(MI);
+    ForwardCalls[F].insert(MI);
   }
 
   // Map a Function to the vector of machine instructions that represents