[MLIR] Improve support for 0-dimensional Affine Maps.

Summary:
Modified AffineMap::get to remove support for the overload which allowed
an ArrayRef of AffineExpr but no context (and gathered the context from a
presumed first entry, resulting in bugs when there were 0 results).

Instead, we support only a ArrayRef and a context, and a version which
takes a single AffineExpr.

Additionally, removed some now needless case logic which previously
special cased which call to AffineMap::get to use.

Reviewers: flaub, bondhugula, rriddle!, nicolasvasilache, ftynse, ulysseB, mravishankar, antiagainst, aartbik

Subscribers: mehdi_amini, jpienaar, burmako, shauheen, antiagainst, arpith-jacob, mgester, lucyrfox, liufengdb, Joonsoo, bader, grosul1, frgossen, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78226

mlir/lib/Dialect/Vector/VectorTransforms.cpp

@@ -1348,9 +1348,7 @@ private:
       auto targetExpr = getAffineDimExpr(idx < index ? idx : idx - 1, ctx);
       results.push_back(targetExpr);
     }
-    // The (...) -> () affine map has its own factory method.
-    return results.empty() ? AffineMap::get(map.getNumDims() - 1, 0, ctx)
-                           : AffineMap::get(map.getNumDims() - 1, 0, results);
+    return AffineMap::get(map.getNumDims() - 1, 0, results, ctx);
   }
 
   // Helper to drop dimension from vector type.