[clang] [llvm] Add support for flag output operand "=@cc" for SystemZ. (PR #125970)

[Ulrich Weigand via cfe-commits]

================
@@ -2837,8 +2837,37 @@ void SelectionDAGBuilder::visitBr(const BranchInst &I) {
       Opcode = Instruction::And;
     else if (match(BOp, m_LogicalOr(m_Value(BOp0), m_Value(BOp1))))
       Opcode = Instruction::Or;
-
-    if (Opcode &&
+    auto &TLI = DAG.getTargetLoweringInfo();
+    bool BrSrlIPM = FuncInfo.MF->getTarget().getTargetTriple().getArch() ==
+                    Triple::ArchType::systemz;
----------------
uweigand wrote:

We really shouldn't have triple checks here in common SelectionDAG code.  If absolutely needed, this should be abstracted behind appropriate target callbacks.

However, I'm wondering if this is indeed needed at all at this point.  Can't we just let common code canonicalize the sequence of if statements into a switch statement, and then recognize the particular form of switch (with input coming from an `IPM`, and only two different switch targets) in platform-specific DAGCombine and directly translate it into a ccmask branch?

https://github.com/llvm/llvm-project/pull/125970