Tests failing with GPUCompiler v0.26.5 and LLVM v7.1

[christiangnrd]

See https://buildkite.com/julialang/metal-dot-jl/builds/993.

[tgymnich]

Here is the root cause: JuliaGPU/llvm-downgrade#4

[maleadt]

Did either of you run into this with a released version of Metal.jl? If not (i.e. if it was strictly tied to LLVM 7 which the latest released Metal.jl isn't compatible with yet) we don't need to rush out a release with #354.

[christiangnrd]

I noticed it because I rebased #321 after addressing review comments and everything was failing when no change I had made should have broken so much so it was most probably tied to LLVM 7 and not in any released version.