Add a key to control llvm jumpThreading pass

JumpThreading pass isn't good for GPU in general as GPU favors
early convergence, where jump threading may dumplicate BBs and
results in more branching code.

Here, add a key to control jumpThreading pass, so that we can
turn it off to see how it goes.

IGC/Compiler/CISACodeGen/ShaderCodeGen.cpp

@@ -1558,7 +1558,8 @@ void OptimizeIR(CodeGenContext* const pContext)
             }
             GFX_ONLY_PASS { mpm.add(new GenUpdateCB()); }
 
-            if (!pContext->m_instrTypes.hasAtomics && !extensiveShader(pContext))
+            if (IGC_IS_FLAG_ENABLED(EnableJumpThreading) &&
+                !pContext->m_instrTypes.hasAtomics && !extensiveShader(pContext))
             {
                 if (pContext->type == ShaderType::OPENCL_SHADER)
                 {
@@ -1597,7 +1598,8 @@ void OptimizeIR(CodeGenContext* const pContext)
 
             // Conditions apply just as above due to problems with atomics
             // (see comment above for details).
-            if (!pContext->m_instrTypes.hasAtomics && !extensiveShader(pContext))
+            if (IGC_IS_FLAG_ENABLED(EnableJumpThreading) &&
+                !pContext->m_instrTypes.hasAtomics && !extensiveShader(pContext))
             {
                 // After lowering 'switch', run jump threading to remove redundant jumps.
                 mpm.add(llvm::createJumpThreadingPass());

IGC/common/igc_flags.h

@@ -173,6 +173,7 @@ DECLARE_IGC_REGKEY(bool, DisableMatchPredAdd,           false, "Setting this to
 DECLARE_IGC_REGKEY(bool, DisableMatchSimpleAdd,         false, "Setting this to 1/true adds a compiler switch to disable simple cmp+and+add optimization", false)
 DECLARE_IGC_REGKEY(bool, DisableMatchPow,               false, "Setting this to 1/true adds a compiler switch to disable log2/mul/exp2 = pow optimization", false)
 DECLARE_IGC_REGKEY(bool, DisableIRVerification,         false, "Setting this to 1/true adds a compiler switch to disable IGC IR verification.", false)
+DECLARE_IGC_REGKEY(bool, EnableJumpThreading,           true,  "Setting this to 1/true adds a compiler switch to enable llvm jumpThreading pass.", true)
 DECLARE_IGC_REGKEY(bool, DisableLoopUnroll,             false, "Setting this to 1/true adds a compiler switch to disable loop unrolling.", true)
 DECLARE_IGC_REGKEY(DWORD, RuntimeLoopUnrolling,         0, "Setting this to switch on/off runtime loop unrolling. 0: default (on), 1: force on, 2: force off", false)
 DECLARE_IGC_REGKEY(bool, DisableBranchSwaping,          false, "Setting this to 1/true adds a compiler switch to disable branch swapping.", false)