Run spirv-opt pass to eliminate dead functions after inlining

Run spirv-opt pass to eliminate dead functions after inlining

The inliner leaves the original functions. If we want the simplicity of
not thinking about functions, we have to remove them.

Change-Id: I0d0c7f8d3de21095b52d86a4c62015287eda32cc
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/27272
Tested-by: Chris Forbes <chrisforbes@google.com>
Presubmit-Ready: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>