CompatHelper: bump compat for LLVM to 8, (keep existing compat) (#2426)

Co-authored-by: CompatHelper Julia <compathelper_noreply@julialang.org>

Author: github-actions[bot]

Project.toml

@@ -61,7 +61,7 @@ ExprTools = "0.1"
 GPUArrays = "10.0.1"
 GPUCompiler = "0.24, 0.25, 0.26"
 KernelAbstractions = "0.9.2"
-LLVM = "6, 7"
+LLVM = "6, 7, 8"
 LLVMLoopInfo = "1"
 LazyArtifacts = "1"
 Libdl = "1"