In order for the Fuchsia source tree to update its
version of SPIRV-Tools to a newer upstream, the
BUILD.gn needs to be slightly altered to take care
of the fact that it can be used with a different
GN //build set of rules and configs than the
Chromium one.

This is done by using |build_with_chromium|, already
defined by //build_overrides/build.gni, to guard
Chromium-specific statements.

+ Add a target to generate spirv-opt which is used by
  Fuchsia to optimize shaders at build time for some
  of its graphics libraries.

Avoids polluting the global namespace with a constant, and moves constructor to .cpp file as is done for spirv-reduce's options.

It is illegal to inline an OpKill instruction into a continue construct because the continue header will no longer dominate the backedge.

This commit adds a check for this, and does not inline.

If we still want to be able to inline a function that contains an OpKill, we can add a new pass that will wrap OpKill instructions into its own function with just the single instruction.

I do not believe that this is a common case right now, so I will not do that yet.

Fixes #2433.

Inlining does not inline functions that have a single return that is in a loop.  This is because the return cannot be replaced by a branch outside of the loop easily.  Merge return knows how to rewrite the function so the return is replaced by a branch.

Fixes #2038.

Adds to spirv-fuzz the option to shrink a sequence of transformations
that lead to an interesting binary to be generated, to find a smaller
sub-sequence of transformations that still lead to an interesting (but
hopefully simpler) binary being generated. The notion of what counts
as "interesting" comes from a user-provided script, the
"interestingness function", similar to the way the spirv-reduce tool
works. The shrinking process will give up after a maximum number of
steps, which can be configured on the command line.

Tests for the combination of fuzzing and shrinking are included, using
a variety of interestingness functions.