Allow isolated regions to form isolated SSA name scopes in the printer.

This will allow for naming values the same as existing SSA values for regions attached to operations that are isolated from above. This fits in with how the system already allows separate name scopes for sibling regions. This name shadowing can be enabled in the custom parser of operations by setting the 'enableNameShadowing' flag to true when calling 'parseRegion'.

%arg = constant 10 : i32
foo.op {
  %arg = constant 10 : i32
}

PiperOrigin-RevId: 264255999

mlir/include/mlir/IR/OpBase.td

@@ -1025,6 +1025,8 @@ class PredOpTrait<string descr, Pred pred> : OpTrait {
 def Broadcastable    : NativeOpTrait<"BroadcastableTwoOperandsOneResult">;
 // X op Y == Y op X
 def Commutative      : NativeOpTrait<"IsCommutative">;
+// Op is isolated from above.
+def IsolatedFromAbove : NativeOpTrait<"IsIsolatedFromAbove">;
 // Op results are float or vectors/tensors thereof.
 def ResultsAreFloatLike : NativeOpTrait<"ResultsAreFloatLike">;
 // Op has no side effect.

mlir/include/mlir/IR/OpImplementation.h

@@ -411,18 +411,24 @@ class OpAsmParser {
 
   /// Parses a region. Any parsed blocks are appended to "region" and must be
   /// moved to the op regions after the op is created. The first block of the
-  /// region takes "arguments" of types "argTypes".
+  /// region takes "arguments" of types "argTypes". If "enableNameShadowing" is
+  /// set to true, the argument names are allowed to shadow the names of other
+  /// existing SSA values defined above the region scope. "enableNameShadowing"
+  /// can only be set to true for regions attached to operations that are
+  /// "IsolatedFromAbove".
   virtual ParseResult parseRegion(Region &region,
                                   ArrayRef<OperandType> arguments,
-                                  ArrayRef<Type> argTypes) = 0;
+                                  ArrayRef<Type> argTypes,
+                                  bool enableNameShadowing = false) = 0;
 
   /// Parses a region if present.
   virtual ParseResult parseOptionalRegion(Region &region,
                                           ArrayRef<OperandType> arguments,
-                                          ArrayRef<Type> argTypes) = 0;
+                                          ArrayRef<Type> argTypes,
+                                          bool enableNameShadowing = false) = 0;
 
-  /// Parse a region argument.  Region arguments define new values; so this also
-  /// checks if values with the same name have not been defined yet.
+  /// Parse a region argument, this argument is resolved when calling
+  /// 'parseRegion'.
   virtual ParseResult parseRegionArgument(OperandType &argument) = 0;
 
   /// Parse zero or more region arguments with a specified surrounding