Add a key EnableVISADCE to control visa dce.

It is off for now, so no functional change from this submit

Author: jgu222

IGC/Compiler/CISACodeGen/CISABuilder.cpp

@@ -3927,6 +3927,11 @@ namespace IGC
             SaveOption(vISA_EnableScalarJmp, false);
         }
 
+        if (IGC_IS_FLAG_ENABLED(EnableVISADCE))
+        {
+            SaveOption(vISA_EnableDCE, true);
+        }
+
         if (IGC_IS_FLAG_ENABLED(DisableCSEL))
         {
             SaveOption(vISA_enableCSEL, false);

IGC/common/igc_flags.def

@@ -34,6 +34,7 @@ DECLARE_IGC_REGKEY(bool, EnableVISADotAll,              false, "Enable VISA DotA
 DECLARE_IGC_REGKEY(bool, EnableVISADebug,               false, "Runs VISA in debug mode, all optimizations disabled", false)
 DECLARE_IGC_REGKEY(DWORD, EnableVISAStructurizer,       1,     "Enable/Disable VISA structurizer. See value defs in igc_flags.hpp.", false)
 DECLARE_IGC_REGKEY(bool, EnableVISAJmpi,                true,  "Enable/Disable VISA generating jmpi (scalar jump).", false)
+DECLARE_IGC_REGKEY(bool, EnableVISADCE,                 false, "Enable/Disable VISA DCE.", false)
 DECLARE_IGC_REGKEY(bool, ForceVISAStructurizer,         false, "Force VISA structurizer for testing. Used on platforms in which we turns off SCF and use UCF by default", false)
 DECLARE_IGC_REGKEY(DWORD, NoMaskWA,                     2,     "Add NoMask WA using software-computed emask flag. Valid value: (two groups) \
                                                                    valid value:  bit[1:0] : which BB to insert - 0(off), 1(divergent BB), \