[SYCL] Disable reassociate pass to reduce register pressure (#3615)

We rely on target specific compiler back-end to run this transformation if it's profitable for particular target.

Author: bader

llvm/lib/Transforms/IPO/PassManagerBuilder.cpp

@@ -435,7 +435,10 @@ void PassManagerBuilder::addFunctionSimplificationPasses(
   if (OptLevel > 1)
     MPM.add(createTailCallEliminationPass()); // Eliminate tail calls
   MPM.add(createCFGSimplificationPass());      // Merge & remove BBs
-  MPM.add(createReassociatePass());           // Reassociate expressions
+  // FIXME: re-association increases variables liveness and therefore register
+  // pressure.
+  if (!SYCLOptimizationMode)
+    MPM.add(createReassociatePass()); // Reassociate expressions
 
   // Do not run loop pass pipeline in "SYCL Optimization Mode". Loop
   // optimizations rely on TTI, which is not accurate for SPIR target.