[llvm] [GISel] Add operands check for G_INSERT_SUBVECTOR and G_EXTRACT_SUBVECTOR in buildInstr (PR #186021)

[Mikael Holmen via llvm-commits]

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mikaelholmen wrote:

Hello @jacquesguan 
DstTy (and several other variables below) are unused in builds without asserts
```
     [...]/llvm/lib/CodeGen/GlobalISel/MachineIRBuilder.cpp:1440:9: error: unused variable 'DstTy' [-Werror,-Wunused-variable]
      1440 |     LLT DstTy = DstOps[0].getLLTTy(*getMRI());
           |         ^~~~~
```

https://github.com/llvm/llvm-project/pull/186021