[NFC][InstCombine] Make check for sret in a vararg function clearer

[aeubanks]

Summary

We're trying to get the parameter index of sret and see if it's part of
a function's varargs.

[rnk]

Yikes. I'm guessing this edge case is actually covered by a test.

This change is NFC, but it looks like a functional change, so consider using the oft-overused NFC tag to clarify that there really is no behavior change.

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

bool InstCombinerImpl::transformConstExprCastCall(CallBase &Call) {
 
   if (FT->getNumParams() < NumActualArgs && FT->isVarArg() &&
       !CallerPAL.isEmpty()) {
     // In this case we have more arguments than the new function type, but we
     // won't be dropping them.  Check that these extra arguments have attributes
     // that are compatible with being a vararg call argument.
     unsigned SRetIdx;
     if (CallerPAL.hasAttrSomewhere(Attribute::StructRet, &SRetIdx) &&
-        SRetIdx > FT->getNumParams())
+        SRetIdx - AttributeList::FirstArgIndex >= FT->getNumParams())
       return false;
   }
 
   // Okay, we decided that this is a safe thing to do: go ahead and start
   // inserting cast instructions as necessary.
   SmallVector<Value *, 8> Args;
   SmallVector<AttributeSet, 8> ArgAttrs;
   Args.reserve(NumActualArgs);