[RISCV] Replace untested code with assert

[frasercrmck]

Summary

We found untested code where negative frame indices were ostensibly
handled despite it being in a block guarded by !MFI.isFixedObjectIndex.

While the implementation of MachineFrameInfo::isFixedObjectIndex
suggests this is possible (i.e., if a frame index was more negative - less than the
number of fixed objects), I couldn't find any test in tree -- for any
target -- where a negative frame index wasn't also a fixed object
offset. I couldn't find a way of creating such a object with the
public MachineFrameInfo creation APIs. Even
MachineFrameInfo::getObjectIndexBegin starts counting at the negative
number of fixed objects, so such frame indices wouldn't be covered by
loops using the provided begin/end methods.

Given all this, an assert that any object encountered in the block is
non-negative seems reasonable.

[kito-cheng]

Pushed to our internal testing system to see what happen, will back to you once I got result :)

[kito-cheng]

Green light on testing :)

[frasercrmck]

rebase

[frasercrmck]

In D126278#3536031, @kito-cheng wrote:

Green light on testing :)

Thanks very much for doing this, it's a big help!

llvm/lib/Target/RISCV/RISCVFrameLowering.cpp

if (hasBP(MF)) {
       // | scalar local variables   | | <----'
       // |--------------------------| -- <-- SP
     }
     // The total amount of padding surrounding RVV objects is described by
     // RVV->getRVVPadding() and it can be zero. It allows us to align the RVV
     // objects to the required alignment.
     if (MFI.getStackID(FI) == TargetStackID::Default) {
       Offset += StackOffset::getFixed(MFI.getStackSize());
-      if (FI < 0)
-        Offset += StackOffset::getFixed(RVFI->getLibCallStackSize());
+      assert(FI >= 0 && "Unhandled negative frame index");
     } else if (MFI.getStackID(FI) == TargetStackID::ScalableVector) {
       // Ensure the base of the RVV stack is correctly aligned: add on the
       // alignment padding.
       int ScalarLocalVarSize = MFI.getStackSize() -
                                RVFI->getCalleeSavedStackSize() +
                                RVFI->getRVVPadding();
       Offset += StackOffset::get(ScalarLocalVarSize, RVFI->getRVVStackSize());
     }