Implement this by flattening outputs and unflattening inputs explicitly.
This allows us to pass down a single struct instead of dealing with the
insanity that would be passing down each flattened member separately.
Remove stage_uniforms_var_id.
Seems to be dead code. Naked uniforms do not exist in SPIR-V for Vulkan,
which this seems to have been intended for. It was also unused elsewhere.
This roughly matches their semantics in SPIR-V and MSL. For `FMin`,
`FMax`, and `FClamp`, and the Metal functions `fast::min()`,
`fast::max()`, and `fast::clamp()`, the result is undefined if any
operand is NaN. For the 'N' operations and their corresponding MSL
`precise::` functions, the result is consistent with IEEE 754 (first
non-NaN wins; result is NaN if all operands are NaN).
We can only do this with 32-bit floats, though, because Metal only
provides these variants for `float`. `half` only has one variant of
these functions that is presumably consistent with IEEE 754. I guess
that's OK; the SPIR-V spec only says that `F{Min,Max,Clamp}` are
undefined for NaNs. Performance might suffer, though.
Certain patterns with OpVectorShuffle (and probably others) will cascade
to so large, that they can cause OOM. After we have observed
force_recompile, don't spend unnecessary memory emitting code which will
never be used.
