Roll SwiftShader 8f20452..a972758

https://swiftshader.googlesource.com/SwiftShader.git/+log/8f20452..a972758

BUG=chromium:904265

TEST=bots

CQ_INCLUDE_TRYBOTS=luci.chromium.try:win_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:android_optional_gpu_tests_rel

Change-Id: I6b5a14c70cc6f3324542923b95bf2f1a87b336cc
Reviewed-on: https://chromium-review.googlesource.com/c/1355524
Reviewed-by: Alexis Hétu <sugoi@chromium.org>
Commit-Queue: Nicolas Capens <capn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612385}