[CPU] Aarch64: actually do reserve-x18 (#19895)

The code was intending to add the `reserve-x18` flag, and it was being
executed... but the `hasFlag` method that it was calling wasn't doing
that it thought it was.

That `hasFlag` method is just a static member that returns `true` if its
string argument starts with a `+`.

There isn't an actual method on `SubtargetFeatures` to check if we
already have a given feature. Anyway, it doesn't matter - in this case,
we don't already have the feature, and even if we did, multiply
specified features are not a problem.

But it really, really matters that we don't accidentally allocate `x18`
again :-)

Fixes #19873.

---------

Signed-off-by: Benoit Jacob <[email protected]>

compiler/plugins/target/LLVMCPU/ResolveCPUAndCPUFeatures.cpp

@@ -81,16 +81,8 @@ void tweakCPUFeatures(const llvm::Triple &triple, std::string &cpu,
                       std::string &cpuFeatures) {
   if (triple.isAArch64()) {
     llvm::SubtargetFeatures targetCpuFeatures(cpuFeatures);
-    // Helper to add a feature if not already present. This check matters as
-    // we check for equality of features to tell whether to generate the error
-    // about implicitly targeting a generic CPU.
-    auto addFeature = [&](const char *feature) {
-      if (!targetCpuFeatures.hasFlag(std::string("+") + feature)) {
-        targetCpuFeatures.AddFeature(feature, true);
-      }
-    };
     // x18 is platform-reserved per the Aarch64 procedure call specification.
-    addFeature("reserve-x18");
+    targetCpuFeatures.AddFeature("reserve-x18", true);
     cpuFeatures = targetCpuFeatures.getString();
   }
 }