[NFC] Add option to disable the scalarizer pass.

[tsymalla]

Summary

This patch adds a new CLI option, which can be used to
disable the scalarizer pass globally.

We found that disabling the scalarizer pass helps with
generating v_pk_* instructions in the AMDGPU backend,
therefore this option should help with experiments which
lead to better understanding of the impact of running the
scalarizer pass in such cases.

[foad]

I think this would be better as an LLPC option that controls whether this pass is added to LLPC's optimization pass pipeline. LLVM doesn't generally provide command line options to enable/disable every single pass.

[tsymalla]

Will add the change as LLPC change.

llvm/lib/Transforms/Scalar/Scalarizer.cpp

 #include <iterator>
 #include <map>
 #include <utility>
 
 using namespace llvm;
 
 #define DEBUG_TYPE "scalarizer"
 
+static cl::opt<bool>
+    DisableScalarizer("disable-scalarizer", cl::init(false), cl::Hidden,
+                      cl::desc("Don't execute the scalarizer pass"));
+
 static cl::opt<bool> ClScalarizeVariableInsertExtract(
     "scalarize-variable-insert-extract", cl::init(true), cl::Hidden,
     cl::desc("Allow the scalarizer pass to scalarize "
              "insertelement/extractelement with variable index"));
 
 // This is disabled by default because having separate loads and stores
 // makes it more likely that the -combiner-alias-analysis limits will be
 // reached.
bool ScalarizerVisitor::finish() {
   Scalarized = false;
 
   RecursivelyDeleteTriviallyDeadInstructionsPermissive(PotentiallyDeadInstrs);
 
   return true;
 }
 
 PreservedAnalyses ScalarizerPass::run(Function &F, FunctionAnalysisManager &AM) {
+  if (DisableScalarizer)
+    return PreservedAnalyses::all();
+
   Module &M = *F.getParent();
   unsigned ParallelLoopAccessMDKind =
       M.getContext().getMDKindID("llvm.mem.parallel_loop_access");
   DominatorTree *DT = &AM.getResult<DominatorTreeAnalysis>(F);
   ScalarizerVisitor Impl(ParallelLoopAccessMDKind, DT, Options);
   bool Changed = Impl.visit(F);
   PreservedAnalyses PA;
   PA.preserve<DominatorTreeAnalysis>();
   return Changed ? PA : PreservedAnalyses::all();
 }