[mlir] [VectorOps] Progressively lower vector.outerproduct to LLVM

Summary:
This replaces the direct lowering of vector.outerproduct to LLVM with progressive lowering into elementary vectors ops to avoid having the similar lowering logic at several places.

NOTE1: with the new progressive rule, the lowered llvm is slightly more elaborate than with the direct lowering, but the generated assembly is just as optimized; still if we want to stay closer to the original, we should add a "broadcast on extract" to shuffle rewrite (rather than special cases all the lowering steps)

NOTE2: the original outerproduct lowering code should now be removed but some linalg test work directly on vector and contain some dead code, so this requires another CL

Reviewers: nicolasvasilache, andydavis1

Reviewed By: nicolasvasilache, andydavis1

Subscribers: mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, liufengdb, Joonsoo, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75956

mlir/lib/Conversion/VectorToLLVM/ConvertVectorToLLVM.cpp

@@ -817,6 +817,7 @@ public:
   }
 };
 
+// TODO(ajcbik): remove this rule once LinAlg tests are cleaned up
 class VectorOuterProductOpConversion : public ConvertToLLVMPattern {
 public:
   explicit VectorOuterProductOpConversion(MLIRContext *context,
@@ -1176,7 +1177,7 @@ struct LowerVectorToLLVMPass : public ModulePass<LowerVectorToLLVMPass> {
 } // namespace
 
 void LowerVectorToLLVMPass::runOnModule() {
-  // Perform progressive lowering of operations on "slices" and
+  // Perform progressive lowering of operations on slices and
   // all contraction operations. Also applies folding and DCE.
   {
     OwningRewritePatternList patterns;