[llvm] [TableGen] Improve handling for dag op names (PR #149248)

[Rahul Joshi via llvm-commits]

================
@@ -16,6 +16,12 @@ def test {
   dag undefSecond  = !con((op 1), (?  2));
   // CHECK: dag undefBoth = (? 1, 2);
   dag undefBoth    = !con((?  1), (?  2));
+  // CHECK: dag namedLHS = (op:$lhs 1, 2);
+  dag namedLHS = !con((op:$lhs 1), (op 2));
+  // CHECK: dag namedRHS = (op 1, 2);
----------------
jurahul wrote:

For these 2, should we choose the rhs name if the lhs one is empty?

https://github.com/llvm/llvm-project/pull/149248