[clang] [CIR] Add cir-simplify pass (PR #138317)

[Bruno Cardoso Lopes via cfe-commits]

================
@@ -0,0 +1,184 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "PassDetail.h"
+#include "mlir/Dialect/Func/IR/FuncOps.h"
+#include "mlir/IR/Block.h"
+#include "mlir/IR/Operation.h"
+#include "mlir/IR/PatternMatch.h"
+#include "mlir/IR/Region.h"
+#include "mlir/Support/LogicalResult.h"
+#include "mlir/Transforms/GreedyPatternRewriteDriver.h"
+#include "clang/CIR/Dialect/IR/CIRDialect.h"
+#include "clang/CIR/Dialect/Passes.h"
+#include "llvm/ADT/SmallVector.h"
+
+using namespace mlir;
+using namespace cir;
+
+//===----------------------------------------------------------------------===//
+// Rewrite patterns
+//===----------------------------------------------------------------------===//
+
+namespace {
+
+/// Simplify suitable ternary operations into select operations.
+///
+/// For now we only simplify those ternary operations whose true and false
+/// branches directly yield a value or a constant. That is, both of the true and
+/// the false branch must either contain a cir.yield operation as the only
+/// operation in the branch, or contain a cir.const operation followed by a
+/// cir.yield operation that yields the constant value.
+///
+/// For example, we will simplify the following ternary operation:
+///
+///   %0 = cir.ternary (%condition, true {
+///     %1 = cir.const ...
+///     cir.yield %1
+///   } false {
+///     cir.yield %2
----------------
bcardosolopes wrote:

This would probably look a bit better:
```
///   %0 = ...
///   %1 = cir.ternary (%condition, true {
///     %2 = cir.const ...
///     cir.yield %2
///   } false {
///     cir.yield %0
```

https://github.com/llvm/llvm-project/pull/138317