[LLVM] Validate generated LLVM module before optimization

[Lunderberg]

Because LLVM's optimizations assume that the generated module is valid, validation should be done before optimization, rather than after. This has the additional benefit of providing error messages from LLVM that more closely relate to the TVM-generated LLVM IR, rather than the optimized LLVM IR.

[tvm-bot]

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_{Generated by tvm-bot}

[Lunderberg]

This was useful for debugging purposes when tracking down a failure reported by llvm::verifyModule on a dev branch (related to #14563).

[Lunderberg]

@tvm-bot rerun