[WebAssembly] Fix feature coalescing

This fixes a problem introduced in #80094. That PR copied negative
features from the TargetMachine to the end of the feature string.
This is not correct, because even if we have a baseline TM of
say `-simd128`, but a function with `+simd128`, the coalesced
feature string must have `+simd128`, not `-sidm128`.

To address the original motivation of that PR, we should instead
explicitly materialize the negative features in the target feature
string, so that explicitly disabled default features are honored.

Unfortunately, there doesn't seem to be any way to actually test
this using llc, because `-mattr` appends the specified features
to the end of the `"target-features"` attribute. I've tested this
locally by making it prepend the features instead.

Author: nikic

llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp

@@ -202,8 +202,7 @@ class CoalesceFeaturesAndStripAtomics final : public ModulePass {
   bool runOnModule(Module &M) override {
     FeatureBitset Features = coalesceFeatures(M);
 
-    std::string FeatureStr =
-        getFeatureString(Features, WasmTM->getTargetFeatureString());
+    std::string FeatureStr = getFeatureString(Features);
     WasmTM->setTargetFeatureString(FeatureStr);
     for (auto &F : M)
       replaceFeatures(F, FeatureStr);
@@ -241,17 +240,14 @@ class CoalesceFeaturesAndStripAtomics final : public ModulePass {
     return Features;
   }
 
-  static std::string getFeatureString(const FeatureBitset &Features,
-                                      StringRef TargetFS) {
+  static std::string getFeatureString(const FeatureBitset &Features) {
     std::string Ret;
     for (const SubtargetFeatureKV &KV : WebAssemblyFeatureKV) {
       if (Features[KV.Value])
         Ret += (StringRef("+") + KV.Key + ",").str();
+      else
+        Ret += (StringRef("-") + KV.Key + ",").str();
     }
-    SubtargetFeatures TF{TargetFS};
-    for (std::string const &F : TF.getFeatures())
-      if (!SubtargetFeatures::isEnabled(F))
-        Ret += F + ",";
     return Ret;
   }