Add exceptions to check_ir_values

[tgymnich]

Allow double for C's default argument promotions used in variable-length argument lists by introducing ir_check_ignore metadata and a allow argument for check_ir_values that is used to allow fpext in Metal.

needed for JuliaGPU/Metal.jl#418

[maleadt]

Are there any other uses for double that are legal? If not, I'd rather we somehow whitelist the printf handling, because the existing error reporting is much more user friendly than the otherwise completely opaque compilation errors as reported by Metal's back-end compiler.

[tgymnich]

Use of double for logging currently consists of fpext and a store. I don't think we can get arround fpext since air.os_log expects doubles.
Maybe we can tag it with some metadata?

[maleadt]

Maybe we can tag it with some metadata?

Yeah, if it survives optimization. Or, alternatively, a function attribute (julia.allow_double) set on the @noinline body of the printf code doing the fpext etc? That may be more robust.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.67%. Comparing base (bbd6124) to head (b77553c).
Report is 2 commits behind head on master.

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@@            Coverage Diff             @@
##           master     #630      +/-   ##
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+ Coverage   71.34%   71.67%   +0.33%     
==========================================
  Files          24       24              
  Lines        3186     3202      +16     
==========================================
+ Hits         2273     2295      +22     
+ Misses        913      907       -6     

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[maleadt]

After some testing, I don't think metadata is going to work. In any case, it's the wrong tool for the job. We could devise more elaborate IR patterns that do survive, but I don't see the value in that.

Let's just not use check_ir_values, which is a simple enough function, and instead roll our own version in metal.jl which ignores the fpext/store. Or, if you feel fancy, maybe a allow_cb callback argument to check_ir_values.

[maleadt]

Maybe add a simple test with pre-defined IR modules to check this behavior (double normally disallowed, only when passed to logging)?