fxc exhibits slow compile performance with dynamic cbuffer indexing.
So when a uniform block contains only one large array member, which is
an array of structures, translate this uniform block to
a StructuredBuffer instead.

Bug: angleproject:3682
TEST=angle_end2end_tests.UniformBufferTest.*
Change-Id: Ife80dba8aae65b761737e095895e00a570230f88
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1782046
Commit-Queue: Xinghua Cao <xinghua.cao@intel.com>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>