[llvm] Use StringRef::operator== instead of StringRef::equals (NFC) (#91441)

I'm planning to remove StringRef::equals in favor of
StringRef::operator==.

- StringRef::operator==/!= outnumber StringRef::equals by a factor of
  70 under llvm/ in terms of their usage.

- The elimination of StringRef::equals brings StringRef closer to
  std::string_view, which has operator== but not equals.

- S == "foo" is more readable than S.equals("foo"), especially for
  !Long.Expression.equals("str") vs Long.Expression != "str".

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -6896,7 +6896,7 @@ Error BitcodeReader::materialize(GlobalValue *GV) {
         MDString *MDS = cast<MDString>(MD->getOperand(0));
         StringRef ProfName = MDS->getString();
         // Check consistency of !prof branch_weights metadata.
-        if (!ProfName.equals("branch_weights"))
+        if (ProfName != "branch_weights")
           continue;
         unsigned ExpectedNumOperands = 0;
         if (BranchInst *BI = dyn_cast<BranchInst>(&I))