[GlobalISel] Fix wrong invocation of `getParamStackAlign` (NFC)

[chill]

Summary

The function template CallLowering::setArgFlags is invoked both
for arguments and return values. In the latter case, it calls
getParamStackAlign with argument index ~0u. Nothing wrong
happens now, as the argument is safely incremented back to 0
inside getParamStackAlign (the type is unsigned), but in
principle it's fragile and may become incorrect.

[chill]

Thanks!

llvm/lib/CodeGen/GlobalISel/CallLowering.cpp

 template <typename FuncInfoTy>
 void CallLowering::setArgFlags(CallLowering::ArgInfo &Arg, unsigned OpIdx,
                                const DataLayout &DL,
                                const FuncInfoTy &FuncInfo) const {
   auto &Flags = Arg.Flags[0];
   const AttributeList &Attrs = FuncInfo.getAttributes();
   addArgFlagsFromAttributes(Flags, Attrs, OpIdx);
 
-  Align MemAlign;
+  Align MemAlign = DL.getABITypeAlign(Arg.Ty);
   if (Flags.isByVal() || Flags.isInAlloca() || Flags.isPreallocated()) {
+    assert(OpIdx >= AttributeList::FirstArgIndex);
     Type *ElementTy = cast<PointerType>(Arg.Ty)->getElementType();
 
     auto Ty = Attrs.getAttribute(OpIdx, Attribute::ByVal).getValueAsType();
     Flags.setByValSize(DL.getTypeAllocSize(Ty ? Ty : ElementTy));
 
     // For ByVal, alignment should be passed from FE.  BE will guess if
     // this info is not there but there are cases it cannot get right.
-    if (auto ParamAlign = FuncInfo.getParamStackAlign(OpIdx - 1))
+    if (auto ParamAlign =
+            FuncInfo.getParamStackAlign(OpIdx - AttributeList::FirstArgIndex))
       MemAlign = *ParamAlign;
-    else if ((ParamAlign = FuncInfo.getParamAlign(OpIdx - 1)))
+    else if ((ParamAlign =
+                  FuncInfo.getParamAlign(OpIdx - AttributeList::FirstArgIndex)))
       MemAlign = *ParamAlign;
     else
       MemAlign = Align(getTLI()->getByValTypeAlignment(ElementTy, DL));
-  } else if (auto ParamAlign = FuncInfo.getParamStackAlign(OpIdx - 1)) {
+  } else if (OpIdx >= AttributeList::FirstArgIndex) {
+    if (auto ParamAlign =
+            FuncInfo.getParamStackAlign(OpIdx - AttributeList::FirstArgIndex))
       MemAlign = *ParamAlign;
-  } else {
-    MemAlign = Align(DL.getABITypeAlign(Arg.Ty));
   }
   Flags.setMemAlign(MemAlign);
   Flags.setOrigAlign(DL.getABITypeAlign(Arg.Ty));
 
   // Don't try to use the returned attribute if the argument is marked as
   // swiftself, since it won't be passed in x0.
   if (Flags.isSwiftSelf())
     Flags.setReturned(false);