Std140 storage layout has some rules for uniform block, must
pad StructuredBuffer element type explicitly to match std140
layout rules when translating std140 uniform block to
StructuredBuffer. In this CL, restrict member types which are
not needed to add paddings according std140 layout rules.

Bug: angleproject:4783
Change-Id: I50130da64ad6865083849bf55b76525f3d2835a8
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2259226
Commit-Queue: Xinghua Cao <xinghua.cao@intel.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jiajia Qin <jiajia.qin@intel.com>