[mlir][vector] Add tests for scalable vectors

Adds tests for scalable vectors in:
  * vector-contract-to-outerproduct-transforms.mlir
Every existing test is duplicated with (fixed-width vectors are replaced
with scalable vectors). One test required a fix in
  * LowerVectorContract.cpp.

This change is a part of a larger effort to enable scalable
vectorisation in Linalg. See this RFC for more context:
  * https://discourse.llvm.org/t/rfc-scalable-vectorisation-in-linalg/

Fixes llvm#67804

mlir/lib/Dialect/Vector/Transforms/LowerVectorContract.cpp

@@ -418,7 +418,7 @@ struct UnrolledOuterProductGenerator
       return v;
     Type promotedType = dstElementType;
     if (vecType)
-      promotedType = VectorType::get(vecType.getShape(), promotedType);
+      promotedType = vecType.clone(promotedType);
     if (isa<FloatType>(dstElementType))
       return rewriter.create<arith::ExtFOp>(loc, promotedType, v);
     return rewriter.create<arith::ExtSIOp>(loc, promotedType, v);