This patch adds a toggle to enable/disable workgroup memory
initialization with OpConstantNull according to the Vulkan extension
VK_KHR_zero_initialize_workgroup_memory. This toggle is by default
enabled when VK_KHR_zero_initialize_workgroup_memory is supported by
the Vulkan driver.
BUG=dawn:1302
TEST=dawn_end2end_tests
Change-Id: Ie04484c2d0944ead082bd22a436b1c52bc7d93bb
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/82400
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>