scan lowering changes

[anchuraj]

Scan reductions are supported in OpenMP with the the help of scan directive. Reduction clause of the for loop/simd directive takes an inscan modifier if scan reduction is specified. With an inscan modifier, the body of the directive should specify a scan directive. This PR implements the lowering logic for scan reductions in workshare loops of OpenMP.
The body of the for loop is splitted into two loops and a scan reduction loop is added in the middle.

size num_iters = <num_iters>;
<type> buffer[num_iters];
for (i: 0..<num_iters>) {
  <input phase>;
  buffer[i] = red;
}
for (int k = 0; k != ceil(log2(num_iters)); ++k) {
  i=pow(2,k)
  for (size cnt = last_iter; cnt >= i; --cnt)
    buffer[cnt] op= buffer[cnt-i];
}
for (0..<num_iters>) {
  red = buffer[i] ;
  <scan phase>;
}

[github-actions]

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff HEAD~1 HEAD --extensions h,cpp -- llvm/include/llvm/Frontend/OpenMP/OMPIRBuilder.h llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp llvm/unittests/Frontend/OpenMPIRBuilderTest.cpp mlir/lib/Target/LLVMIR/Dialect/OpenMP/OpenMPToLLVMIRTranslation.cpp

View the diff from clang-format here.

diff --git a/mlir/lib/Target/LLVMIR/Dialect/OpenMP/OpenMPToLLVMIRTranslation.cpp b/mlir/lib/Target/LLVMIR/Dialect/OpenMP/OpenMPToLLVMIRTranslation.cpp
index e18ef5233..2ef6520ba 100644
--- a/mlir/lib/Target/LLVMIR/Dialect/OpenMP/OpenMPToLLVMIRTranslation.cpp
+++ b/mlir/lib/Target/LLVMIR/Dialect/OpenMP/OpenMPToLLVMIRTranslation.cpp
@@ -2817,7 +2817,7 @@ convertOmpLoopNest(Operation &opInst, llvm::IRBuilderBase &builder,
           wsloopOp.getReductionMod() && (wsloopOp.getReductionMod().value() ==
                                          mlir::omp::ReductionModifier::inscan);
       if (isInScanRegion) {
-        //TODO: Handle nesting if Scan loop is nested in a loop
+        // TODO: Handle nesting if Scan loop is nested in a loop
         assert(loopOp.getNumLoops() == 1);
         llvm::Expected<SmallVector<llvm::CanonicalLoopInfo *>> loopResults =
             ompBuilder->createCanonicalScanLoops(