[WebAssembly] Fix feature coalescing (#110647)

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 should have
`+simd128`, not `-simd128`.

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

@@ -205,8 +205,7 @@ 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);
@@ -244,17 +243,14 @@ 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;
   }