[clang] [llvm] [WIP][APX] Allow EGPR registers used as non-volatile registers (PR #173224)

[via cfe-commits]

azwolski wrote:

I think we may need add this
 ```cpp                                                                       
  // Options.td                                                                   
  def mapx_features_egpr_nv_EQ : Joined<["-"], "mapx-features=egpr-nv:">,         
  Group<m_x86_Features_Group>,                                                    
      HelpText<"Set number of non-volatile registers of APX EGPR feature">;       
                                                                                  
  // X86.cpp      
      // Handle -mapx-features=egpr-nv:N                                          
      if (A->getOption().matches(options::OPT_mapx_features_egpr_nv_EQ)) {        
        Features.push_back("+egpr");                                              
        continue;                                                                 
      }                                                                           
                                                                                  
  // CommonArgs.cpp                                                               
    if (Arg *A = Args.getLastArg(options::OPT__SLASH_apx_features_egprnv,         
                                  options::OPT_mapx_features_egpr_nv_EQ)) {       
      StringRef Value = A->getValue();                                            
      CmdArgs.push_back("-mllvm");                                                
      CmdArgs.push_back(Args.MakeArgString("-apx-egpr-csr=" + Value));            
    }                                                                             
````

https://github.com/llvm/llvm-project/pull/173224