fix: MSVC SIMD compiling

[EduMenges]

I have found issues when compiling the SIMD targets for MSVC.

Firstly, the target MNNAVX requires the AVX2 instruction set. While this is correctly set when using Clang:

target_compile_options(MNNAVX PRIVATE -mavx2 -DMNN_X86_USE_ASM)

It is not correct when using MSVC, which is still set to AVX. This prevents correct compiling.

The same is true for the MNNSSE target, which requires the AVX arch (the SSE arch is ignored in x64).

Lastly, there is a weird 256 bit dereference as if it is a struct, but in MSVC it is an opaque type, thus it should be dereferenced using SIMD intrinsics. I've updated the code so that the GCC/MSVC distinction is not necessary.

The fact that MNN's CI for Windows does not catch these errors makes me think the SIMD parts are not getting compiled during CI.

[jxt1234]

What's your version of Vision Studio. It seems not meet compile error for 2017 and later.

[EduMenges]

What's your version of Vision Studio. It seems not meet compile error for 2017 and later.

It's the 2022 Community Edition, more precisely version 17.12.3.