[llvm] Add a TI_WITH_LLVM option

[AmesingFlank]

This PR adds a TI_WITH_LLVM option, so that one can optionally disable compiling with LLVM. (For example, I'm trying to emscripten the entire taichi project, and LLVM doesn't like that).

TI_WITH_LLVM is defaulted to ON, so this PR won't affect any of our existing CI/CD/build tools.

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[qiao-bo]

Thanks. A quick grep into our cpp and h files shows 263 occurrences of endif, many of which are small blocks. I feel like this may hinder readability in the long term.

[AmesingFlank]

Thanks. A quick grep into our cpp and h files shows 263 occurrences of endif, many of which are small blocks. I feel like this may hinder readability in the long term.

I agree :(( The right thing to do here would be to:

• Move taichi/llvm taichi/codegen (and some other stuff such as SNodeBufferManager) into a folder in taichi/backend, and use cmake to omit compiling that folder
• Keep refactoring program.cpp until it is fully backend-independent (i.e. remove all the llvm stuff).

I'm happy to create an issue to track this, but it would be a rather big effort.

[qiao-bo]

Thanks. A quick grep into our cpp and h files shows 263 occurrences of endif, many of which are small blocks. I feel like this may hinder readability in the long term.

I agree :(( The right thing to do here would be to:

• Move taichi/llvm taichi/codegen (and some other stuff such as SNodeBufferManager) into a folder in taichi/backend, and use cmake to omit compiling that folder
• Keep refactoring program.cpp until it is fully backend-independent (i.e. remove all the llvm stuff).

I'm happy to create an issue to track this, but it would be a rather big effort.

Indeed a big effort. Having a llvm folder in taichi/backend seems reasonable to me. This can also move some llvm-specific code from device.h/cpp. For now, i think this is fine.

[qiao-bo]

LGTM!