[GlobalISel] Fix incorrect setting of ValNo when splitting

[ehjogab]

Summary

Before, for each original argument i, ValNo was set to i + PartIdx, but
ValNo is intended to reflect the index of the value before splitting.
Hence, ValNo should always be set to i and not consider the PartIdx.

[ehjogab]

I should mention that this is required for split arguments to work for our target, and it is possible that we're doing something wrong on our end. I don't see it, however, so I decided to submit this for review in hopes that someone either can tell me whether our backend is incorrectly implemented or this is indeed a bug in global-isel.

[arsenm]

LGTM. There's definitely something wrong with the call lowering API where the argument splitting responsibility seems to be randomly split between the generic and target code

[ehjogab]

Rebase to latest master

[ehjogab]

Rebased to latest (forgot to pull)

[arsenm]

There's no changes though

[uabelho]

I submited this for ehjogab

llvm/lib/CodeGen/GlobalISel/CallLowering.cpp

if (Handler.isIncomingArgumentHandler()) {
             Flags.setSplit();
           } else {
             Flags.setOrigAlign(Align(1));
             if (Part == NumParts - 1)
               Flags.setSplitEnd();
           }
           Args[i].Regs.push_back(Reg);
           Args[i].Flags.push_back(Flags);
-          if (Handler.assignArg(i + Part, NewVT, NewVT, CCValAssign::Full,
+          if (Handler.assignArg(i, NewVT, NewVT, CCValAssign::Full,
                                 Args[i], Args[i].Flags[Part], CCInfo)) {
             // Still couldn't assign this smaller part type for some reason.
             return false;
           }
         }
       }
     } else {
       // Handling an outgoing arg that might need to be split.
if (Handler.isIncomingArgumentHandler()) {
           Flags.setSplit();
         } else {
           Flags.setOrigAlign(Align(1));
           if (PartIdx == NumParts - 1)
             Flags.setSplitEnd();
         }
         Args[i].Regs.push_back(Unmerge.getReg(PartIdx));
         Args[i].Flags.push_back(Flags);
-        if (Handler.assignArg(i + PartIdx, NewVT, NewVT, CCValAssign::Full,
+        if (Handler.assignArg(i, NewVT, NewVT, CCValAssign::Full,
                               Args[i], Args[i].Flags[PartIdx], CCInfo))
           return false;
       }
     }
   }
 
   for (unsigned i = 0, e = Args.size(), j = 0; i != e; ++i, ++j) {
     assert(j < ArgLocs.size() && "Skipped too many arg locs");