In some cases, the compiler decided to emit continue block first,
which invalidated the expressions used by the condition.
Parameters to functions can be evaluated in any order which caused
"random" behavior.
Add to suite of MSL tests and references any existing GLSL tests
that successfully convert GLSL->SPIRV->MSL and compile as MSL.
test_shaders_helper() ignores hidden files that start with '.',
to avoid accidentally finding hidden OSX files such as .DS_Store.
Use xcrun to compile MSL shaders instead of hard-coded path to Metal compiler.
Wrap calls to xcrun in exception handling to ignore if Xcode not installed.
For MSL tests, move call to validate_shader_msl() to after call to
regression_check() to allow a converted MSL shader to be saved for
manual review even if it doesn't successfully compile as MSL.
If we run on a system with Xcode installed to a default path, run Xcode
Metal shader compiler to validate the generated MSL shader.
This uncovers an issue in the existing MSL test - MSL backend currently
does not auto-assign attribute locations, which means that translating
GLSL shader without location layout produces an invalid MSL which
generates "error: 'attribute' index '0' is used more than once".
