[mlir][vector][nfc] Add clarification on "dim-1" bcast

Adds a small note to VectorOps.td on what "dim-1" broadcast is. Also
updates comments to consistently use quotes, i.e.

* "dim-1" broadcasting instead of dim-1 broadcasting.

This way it is clear that we are referring to "stretching" one of the
trailing dims rather than e.g. broadcasting a dim at idx 1.

Author: banach-space

mlir/include/mlir/Dialect/Vector/IR/VectorOps.td

@@ -373,8 +373,9 @@ def Vector_BroadcastOp :
 
     The source operand is duplicated over all the missing leading dimensions
     and stretched over the trailing dimensions where the source has a non-equal
-    dimension of 1. These rules imply that any scalar broadcast (k=0) to any
-    shaped vector with the same element type is always legal.
+    dimension of 1 (stretching a trailing dimension is also referred to as
+    "dim-1" broadcasting). These rules imply that any scalar broadcast (k=0) to
+    any shaped vector with the same element type is always legal.
 
     Example:
 
@@ -396,7 +397,7 @@ def Vector_BroadcastOp :
 
     /// Broadcast `value` to a vector of `dstShape`, knowing that exactly the
     /// `broadcastedDims` dimensions in the dstShape are broadcasted.
-    /// This requires (and asserts) that the broadcast is free of dim-1
+    /// This requires (and asserts) that the broadcast is free of "dim-1"
     /// broadcasting.
     /// Since vector.broadcast only allows expanding leading dimensions, an extra
     /// vector.transpose may be inserted to make the broadcast possible.

mlir/lib/Dialect/Vector/IR/VectorOps.cpp

@@ -2355,7 +2355,7 @@ computeBroadcastedUnitDims(ArrayRef<int64_t> srcShape,
   for (auto [s1, s2] :
        llvm::zip_equal(srcShape, dstShape.drop_front(rankDiff))) {
     if (s1 != s2) {
-      assert(s1 == 1 && "expected dim-1 broadcasting");
+      assert(s1 == 1 && "expected \"dim-1\" broadcasting");
       res.insert(dstDim);
     }
     ++dstDim;
@@ -2374,7 +2374,7 @@ llvm::SetVector<int64_t> BroadcastOp::computeBroadcastedUnitDims() {
 
 /// Broadcast `value` to a vector of `dstShape`, knowing that exactly the
 /// `broadcastedDims` dimensions in the dstShape are broadcasted.
-/// This requires (and asserts) that the broadcast is free of dim-1
+/// This requires (and asserts) that the broadcast is free of "dim-1"
 /// broadcasting.
 /// Since vector.broadcast only allows expanding leading dimensions, an extra
 /// vector.transpose may be inserted to make the broadcast possible.
@@ -2460,10 +2460,10 @@ Value BroadcastOp::createOrFoldBroadcastOp(
   // 3.c. Append the srcShape.
   llvm::append_range(broadcastShape, srcVectorType.getShape());
 
-  // Ensure there are no dim-1 broadcasts.
+  // Ensure there are no "dim-1" broadcasts.
   assert(::computeBroadcastedUnitDims(srcVectorType.getShape(), broadcastShape)
              .empty() &&
-         "unexpected dim-1 broadcast");
+         "unexpected \"dim-1\" broadcast");
 
   VectorType broadcastType = VectorType::get(broadcastShape, elementType);
   assert(vector::isBroadcastableTo(value.getType(), broadcastType) ==