script/sign: avoid duplicated signature verification after signing

`ProduceSignature` already calls `VerifyScript` internally as last step in
order to check whether the signature data is complete. If and only if that is
the case, the `complete` field of the `SignatureData` is set accordingly and
there is no need then to verify the script after again, as we already know that
it would succeed.

This leads to a rough ~20% speed-up for `SignTransaction` for single-input
ECDSA or Taproot inputs, according to the `SignTransaction{ECDSA,Taproot}`
benchmarks.

Author: theStack

src/script/sign.cpp

@@ -831,7 +831,7 @@ bool SignTransaction(CMutableTransaction& mtx, const SigningProvider* keystore,
         }
 
         ScriptError serror = SCRIPT_ERR_OK;
-        if (!VerifyScript(txin.scriptSig, prevPubKey, &txin.scriptWitness, STANDARD_SCRIPT_VERIFY_FLAGS, TransactionSignatureChecker(&txConst, i, amount, txdata, MissingDataBehavior::FAIL), &serror)) {
+        if (!sigdata.complete && !VerifyScript(txin.scriptSig, prevPubKey, &txin.scriptWitness, STANDARD_SCRIPT_VERIFY_FLAGS, TransactionSignatureChecker(&txConst, i, amount, txdata, MissingDataBehavior::FAIL), &serror)) {
             if (serror == SCRIPT_ERR_INVALID_STACK_OPERATION) {
                 // Unable to sign input and verification failed (possible attempt to partially sign).
                 input_errors[i] = Untranslated("Unable to sign input, invalid stack size (possibly missing key)");