llvm-profgen: Parse command line arguments after initializing targets

[tstellar]

Summary

I am experimenting with turning backends into loadable modules and in
that scenario, target specific command line arguments won't be available
until after the targets are initialized.

Also, most other tools initialize targets before parsing arguments.

[wlei]

LGTM, Thanks for woking on this @tstellar

llvm/tools/llvm-profgen/llvm-profgen.cpp

 //===- llvm-profgen.cpp - LLVM SPGO profile generation tool -----*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
BinaryFilenames("binary", cl::value_desc("binary"), cl::OneOrMore,
                     cl::desc("Path of profiled binary files"));
 
 using namespace llvm;
 using namespace sampleprof;
 
 int main(int argc, const char *argv[]) {
   InitLLVM X(argc, argv);
 
-  cl::ParseCommandLineOptions(argc, argv, "llvm SPGO profile generator\n");
-
   // Initialize targets and assembly printers/parsers.
   InitializeAllTargetInfos();
   InitializeAllTargetMCs();
   InitializeAllDisassemblers();
 
+  cl::ParseCommandLineOptions(argc, argv, "llvm SPGO profile generator\n");
+
   // Load binaries and parse perf events and samples
   PerfReader Reader(BinaryFilenames);
   Reader.parsePerfTraces(PerfTraceFilenames);
 
   std::unique_ptr<ProfileGenerator> Generator = ProfileGenerator::create(
       Reader.getBinarySampleCounters(), Reader.getPerfScriptType());
   Generator->generateProfile();
   Generator->write();
 
   return EXIT_SUCCESS;
 }