[mlir][SCF] Disallow multiple blocks in scf.if "else" region

[springerm]

Summary

The "then" region allows only a single block. The same should be the case for the "else" region.

[jreiffers]

Thanks!

mlir/include/mlir/Dialect/SCF/IR/SCFOps.td

let description = [{
     ```mlir
     scf.if %b  {
       ...
     } else {
       ...
     }
     ```
 
-    `scf.if` may also return results that are defined in its regions. The
-    values defined are determined by which execution path is taken.
+    `scf.if` may also produce results. Which values are returned depends on
+    which execution path is taken.
 
     Example:
 
     ```mlir
     %x, %y = scf.if %b -> (f32, f32) {
       %x_true = ...
       %y_true = ...
       scf.yield %x_true, %y_true : f32, f32
     } else {
       %x_false = ...
       %y_false = ...
       scf.yield %x_false, %y_false : f32, f32
     }
     ```
 
-    `scf.if` regions are always terminated with "scf.yield". If "scf.if"
-    defines no values, the "scf.yield" can be left out, and will be inserted
-    implicitly. Otherwise, it must be explicit.
-    Also, if "scf.if" defines one or more values, the 'else' block cannot be
-    omitted.
+    The "then" region has exactly 1 block. The "else" region may have 0 or 1
+    block. In case the `scf.if` produces results, the "else" region must also
+    have exactly 1 block.
+
+    The blocks are always terminated with `scf.yield`. If `scf.if` defines no
+    values, the `scf.yield` can be left out, and will be inserted implicitly.
+    Otherwise, it must be explicit.
 
     Example:
 
     ```mlir
     scf.if %b  {
       ...
     }
     ```
+
+    The types of the yielded values must match the result types of the
+    `scf.if`.
   }];
   let arguments = (ins I1:$condition);
   let results = (outs Variadic<AnyType>:$results);
-  let regions = (region SizedRegion<1>:$thenRegion, AnyRegion:$elseRegion);
+  let regions = (region SizedRegion<1>:$thenRegion,
+                        MaxSizedRegion<1>:$elseRegion);
 
   let skipDefaultBuilders = 1;
   let builders = [
     OpBuilder<(ins "TypeRange":$resultTypes, "Value":$cond)>,
     OpBuilder<(ins "Value":$cond, "bool":$withElseRegion)>,
     OpBuilder<(ins "TypeRange":$resultTypes, "Value":$cond,
       "bool":$withElseRegion)>,
     OpBuilder<(ins "Value":$cond,

mlir/include/mlir/IR/OpBase.td

class SizedRegion<int numBlocks> : Region<
   CPred<"::llvm::hasNItems($_self, " # numBlocks # ")">,
   "region with " # numBlocks # " blocks">;
 
 // A region with at least the given number of blocks.
 class MinSizedRegion<int numBlocks> : Region<
   CPred<"::llvm::hasNItemsOrMore($_self, " # numBlocks # ")">,
   "region with at least " # numBlocks # " blocks">;
 
+// A region with at most the given number of blocks.
+class MaxSizedRegion<int numBlocks> : Region<
+  CPred<"::llvm::hasNItemsOrLess($_self, " # numBlocks # ")">,
+  "region with at most " # numBlocks # " blocks">;
+
 // A variadic region constraint. It expands to zero or more of the base region.
 class VariadicRegion<Region region>
   : Region<region.predicate, region.summary>;
 
 //===----------------------------------------------------------------------===//
 // Successor definitions
 //===----------------------------------------------------------------------===//