Add a compiler perf test that's targeted at AST traversers. It
triggers many traversers that mutate the AST and run multiple
iterations particularly on the HLSL backend.

BUG=angleproject:827
TEST=angle_perftests

Change-Id: I75d89e8ae0fd7959f2c7fbb133c13ccde22abc37
Reviewed-on: https://chromium-review.googlesource.com/904622Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>