[llvm] [NFCI][TableGen][DecoderEmitter] Cull `TryDecode` handling when possible (PR #142974)

[Rahul Joshi via llvm-commits]

jurahul wrote:

Here are the code size reductions based on a modified version of this:

```
Target                                                   Old Size        New Size  %  reduction
build/lib/libLLVMAArch64Disassembler.so.21.0git             256656          256656          0.00
build/lib/libLLVMAMDGPUDisassembler.so.21.0git              813000          808168          0.59
build/lib/libLLVMARCDisassembler.so.21.0git                  44816           43536          2.86
build/lib/libLLVMARMDisassembler.so.21.0git                 281744          278808          1.04
build/lib/libLLVMAVRDisassembler.so.21.0git                  36040           34496          4.28
build/lib/libLLVMBPFDisassembler.so.21.0git                  26248           23168         11.73
build/lib/libLLVMCSKYDisassembler.so.21.0git                 55960           53632          4.16
build/lib/libLLVMHexagonDisassembler.so.21.0git             115952          113416          2.19
build/lib/libLLVMLanaiDisassembler.so.21.0git                24360           21008         13.76
build/lib/libLLVMLoongArchDisassembler.so.21.0git            58584           56168          4.12
build/lib/libLLVMM68kDisassembler.so.21.0git                 57264           53880          5.91
build/lib/libLLVMMSP430Disassembler.so.21.0git               28896           28440          1.58
build/lib/libLLVMMipsDisassembler.so.21.0git                123128          120568          2.08
build/lib/libLLVMPowerPCDisassembler.so.21.0git              80656           78096          3.17
build/lib/libLLVMRISCVDisassembler.so.21.0git               154080          150200          2.52
build/lib/libLLVMSparcDisassembler.so.21.0git                42040           39568          5.88
build/lib/libLLVMSystemZDisassembler.so.21.0git              97056           94552          2.58
build/lib/libLLVMVEDisassembler.so.21.0git                   83944           81352          3.09
build/lib/libLLVMWebAssemblyDisassembler.so.21.0git          25280           25280          0.00
build/lib/libLLVMX86Disassembler.so.21.0git                2920624         2920624          0.00
build/lib/libLLVMXCoreDisassembler.so.21.0git                48320           44288          8.34
build/lib/libLLVMXtensaDisassembler.so.21.0git               42248           35840         15.17
```

https://github.com/llvm/llvm-project/pull/142974