Bump LLVM downgrader (#507)

v0.6 includes fixes for constant values and known intrinsics.

Author: maleadt

Project.toml

@@ -44,7 +44,7 @@ GPUArrays = "11.1"
 GPUCompiler = "0.26, 0.27, 1"
 KernelAbstractions = "0.9.1"
 LLVM = "7.2, 8, 9"
-LLVMDowngrader_jll = "0.5"
+LLVMDowngrader_jll = "0.6"
 ObjectFile = "0.4"
 ObjectiveC = "2.1, 3"
 PrecompileTools = "1"