[SPIRV][NFC] Use DenseMap's lookup instead of find (llvm#164237)

[lookup](https://llvm.org/doxygen/classllvm_1_1DenseMapBase.html#a0b2ca98dc28c61793ff5c90d23e5f14e)
does a find and returns the default if no matching element was found.

Author: jmmartinez

llvm/lib/Target/SPIRV/SPIRVModuleAnalysis.h

@@ -169,9 +169,7 @@ struct ModuleAnalysisInfo {
 
   MCRegister getFuncReg(const Function *F) {
     assert(F && "Function is null");
-    auto FuncPtrRegPair = FuncMap.find(F);
-    return FuncPtrRegPair == FuncMap.end() ? MCRegister()
-                                           : FuncPtrRegPair->second;
+    return FuncMap.lookup(F);
   }
   MCRegister getExtInstSetReg(unsigned SetNum) { return ExtInstSetMap[SetNum]; }
   InstrList &getMSInstrs(unsigned MSType) { return MS[MSType]; }