[Analysis] Fix gcc warnings about unused variables [NFC]

gcc warned with:
 [236/4788] Building CXX object lib/Analysis/CMakeFiles/LLVMAnalysis.dir/LazyValueInfo.cpp.o
 ../lib/Analysis/LazyValueInfo.cpp: In member function 'void llvm::LazyValueInfo::forgetValue(llvm::Value*)':
 ../lib/Analysis/LazyValueInfo.cpp:1978:13: warning: unused variable 'Impl' [-Wunused-variable]
  1978 |   if (auto *Impl = getImpl())
       |             ^~~~
 ../lib/Analysis/LazyValueInfo.cpp: In member function 'void llvm::LazyValueInfo::eraseBlock(llvm::BasicBlock*)':
 ../lib/Analysis/LazyValueInfo.cpp:1983:13: warning: unused variable 'Impl' [-Wunused-variable]
  1983 |   if (auto *Impl = getImpl())
       |             ^~~~
 ../lib/Analysis/LazyValueInfo.cpp: In member function 'void llvm::LazyValueInfo::clear()':
 ../lib/Analysis/LazyValueInfo.cpp:1988:13: warning: unused variable 'Impl' [-Wunused-variable]
  1988 |   if (auto *Impl = getImpl())
       |             ^~~~
 ../lib/Analysis/LazyValueInfo.cpp: In member function 'void llvm::LazyValueInfo::printLVI(llvm::Function&, llvm::DominatorTree&, llvm::raw_ostream&)':
 ../lib/Analysis/LazyValueInfo.cpp:1993:13: warning: unused variable 'Impl' [-Wunused-variable]
  1993 |   if (auto *Impl = getImpl())
       |             ^~~~

Use the locals instead of calling getImpl() again.