[X86] Remove redundant declarations (NFC) (llvm#166107)

In C++17, static constexpr members are implicitly inline, so they no
longer require an out-of-line definition.

Identified with readability-redundant-declaration.

Author: kazutakahirata

llvm/lib/Target/X86/X86LoadValueInjectionLoadHardening.cpp

@@ -227,9 +227,6 @@ struct DOTGraphTraits<MachineGadgetGraph *> : DefaultDOTGraphTraits {
 
 } // end namespace llvm
 
-constexpr MachineInstr *MachineGadgetGraph::ArgNodeSentinel;
-constexpr int MachineGadgetGraph::GadgetEdgeSentinel;
-
 char X86LoadValueInjectionLoadHardeningPass::ID = 0;
 
 void X86LoadValueInjectionLoadHardeningPass::getAnalysisUsage(