R600: fix DenseMap with pointer key iteration in the structurizer

Use a MapVector on types where the iteration order matters.
Otherwise we doesn't always produce a deterministic output.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177999 91177308-0d34-0410-b5e6-96231b3b80d8