SPIR-V postprocessing: WEB case only needs CFG mods

Fix compileGLSLZeroCopy accidental minification

The SPIR-V post-processing to discover capabilities and
extensions does not apply to WebGPU compilation.  So don't include
that code.

This reclaims some of the code space added by #1943

Fix #1959 by reverting 7b0e2363 to put files in two places.

Add deprecation for SPIRV/ install folder location.

This Cl updates the README.md to reference the deprecation date for the
current SPIRV/ install location.

Issue #1964

Remove std::move that is breaking chromium roll

This is causing the following error:

moving a temporary object prevents copy elision
[-Werror,-Wpessimizing-move]

Add a couple missing headers to the GN build.

Fix warning: size_t->int implicit cast

Detected with ANGLE's export_targets script.

Add option ENABLE_CTEST to skip testing

More aggressively prune unreachable code as follows.
When no control flow edges reach a merge block or continue target:
- delete their contents so that:
  - a merge block becomes OpLabel, then OpUnreachable
  - a continue target becomes OpLabel, then an OpBranch back to the
    loop header
- any basic block which is dominated by such a merge block or continue
  target is removed as well.
- decorations targeting the removed instructions are removed.

Enables the SPIR-V builder post-processing step the GLSLANG_WEB case.

Update spirv-tools known good.

Make  buildbot licenses consistent with the rest of the code

Web: Extend with compute stage, SSBOs, and atomics on SSBO memory.

The Fuchsia platform build and the Chromium one use a completely
different set of configs to specify default warnings. The previous
version of BUILD.gn worked with Chromium, but failed to build
with Fuchsia.

This CL fixes the issue by identifying said configs and reusing
them consistently.

Change-Id: I7de526a57d2f14eb93c03e06401d2c3059d35e9a

Looks like will add about 1K compressed size to executable footprint.

Add Bazel build files and bots for them