[clang] [CIR] Add if statement support (PR #134333)

[via cfe-commits]

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@@ -263,6 +264,72 @@ static void terminateBody(CIRGenBuilderTy &builder, mlir::Region &r,
     b->erase();
 }
 
+mlir::LogicalResult CIRGenFunction::emitIfStmt(const IfStmt &s) {
+  mlir::LogicalResult res = mlir::success();
+  // The else branch of a consteval if statement is always the only branch
+  // that can be runtime evaluated.
+  const Stmt *ConstevalExecuted;
+  if (s.isConsteval()) {
+    ConstevalExecuted = s.isNegatedConsteval() ? s.getThen() : s.getElse();
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Andres-Salamanca wrote:

I added the test to a new `if-consteval.cpp`, based on [this file](https://github.com/llvm/clangir/blob/main/clang/test/CIR/CodeGen/if-consteval.cpp).  Since we don't support `call` op yet, I tweaked the test slightly.  
Also added the extra `RUN:` line.


https://github.com/llvm/llvm-project/pull/134333