[llvm] [TableGen][DecoderEmitter] Add option to emit type-specialized `decodeToMCInst` (PR #146593)

[Rahul Joshi via llvm-commits]

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@@ -32,44 +32,6 @@ class MCOperand;
 class MCSubtargetInfo;
 class Twine;
 
-// Exposes an interface expected by autogenerated code in
-// FixedLenDecoderEmitter
-class DecoderUInt128 {
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jurahul wrote:

I don't think some of these requirements are valid anymore. For example, extractBitsAsZExtValue is not required, since the code works with bitset<> types. which do not provide this. The bitwise operators are natively supported by all the inferred types, so a helper class is not needed. So beyond potentially generating constructors for bitset types, I am not sure what additional utility this will provide. 

Also note that these requirements are for the `InsnType` template type, which will eventually go away once generation of templated code is deprecated.

https://github.com/llvm/llvm-project/pull/146593