When the "if" qualifier is added to a directive, the directive is
skipped if the shader->minimum_shader_model is not included in the
range.
This can be used on the "probe" directives for tests that have different
expected results on different shader models, without having to resort to
[require] blocks.
The location of dxcompiler should be set during configuration with
'DXCOMPILER_LIBS=-L/path/to/dxcompiler', and then at runtime with
LD_LIBRARY_PATH, WINEPATH or PATH as applicable.
A new 'fail(sm<6)' decoration is needed on many shader declarations
because dxcompiler succeeds on many shaders which fail with fxc. The
opposite case is less common and is flagged with 'fail(sm>=6)'. A few
tests cause dxcompiler to crash or hang, so these are avoided using
[require], which now skips tests until reset instead of exiting. Also,
'todo(sm<6)' and 'todo(sm>=6)' are used to separate checking of results.
