[llvm] [LLVM][MC] Add support to cull inactive decoders in decoder emitter (PR #154865)

[Rahul Joshi via llvm-commits]

jurahul wrote:

> > ```
> > template <typename T>
> > constexpr bool isSupportedInsnBitWidth(uint32_t N) { return false; }
> > ```
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> > This will allow using the same C++ type for > 1 bitwidths. Then we would need to remove the current specializations in MCDecoder.h as they assume a specific usage and then backends can choose. Not sure if that generalization is needed.
> 
> LMK if this is something to pursue. I suspect it may not be too much effort to do this and offers additional flexibility for backends.

I did and now RISCV can continue using uint64_t for 48-bit insts. Will update the PR soon

https://github.com/llvm/llvm-project/pull/154865