[WebAssembly] SIMD intrinsics should be constant folded

[tlively]

The WebAssembly target supports a large number of SIMD intrinsics, many of which are generated from clang builtins. None of these intrinsics can be constant folded, however, which may significantly hinder optimizations when the builtins are used, including when they are used indirectly via the wasm_simd128.h intrinsic header.

[llvmbot]

@llvm/issue-subscribers-backend-webassembly

[badumbatish]

i'm investigating this. A few examples in godbolt https://godbolt.org/z/s1PTnz47s.

I'm not sure if implementations for this should go into WebAssemblyISelLowering.cpp or another place??

[tlively]

I believe the implementations should go in lib/Analysis/ConstantFolding.cpp. Here's an example where some non-SIMD Wasm intrinsics are constant folded: https://github.com/llvm/llvm-project/blob/main/llvm/lib/Analysis/ConstantFolding.cpp#L2279-L2280