[clang] [clang][RISCV] Fix crash on VLS calling convention (PR #145489)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic commented:

I'm not really happy with the way the code is organized for "coerced" load/store involving scalable types.

At the point where we're analyzing the calling convention; we have a bunch of information: exactly what kind of coercion we're doing, whether the sizes match, whether there are any unusual types involved.  We're throwing away that information, and then trying to re-derive it later.

So I think:

- The target code should compute whether the sizes are guaranteed to match.  If they do, "coerced" load/store simplifies to just a plain load/store, and we can skip all these complicated computations involving the sizes of the source/destination.
- If some types need target-specific code to lower arguments, that should be its own ABIArgInfo::Kind, and the target-independent code should call into a target-specific implementation in clang/lib/CodeGen/Targets/RISCV.cpp.

https://github.com/llvm/llvm-project/pull/145489