scons/windows: Fix build with LLVM>=8
Even though LLVMAggressiveInstCombine library is not directly used by Mesa3D, it is used by LLVMipo which is part of coroutines component. Without it build fails with an error like the one from this MSYS2 Mingw-w64 CI test: https://ci.appveyor.com/project/pal1000/mesa/builds/27865129
Fixes eebe091d ("scons/windows: Enable compute shaders when possible.")