Metal: report unsupported atomics when lowering them

[maleadt]

validate_ir rejects the atomics that Metal can't express. When validation is disabled, e.g. by code_native and the other reflection functions, lower_atomics! used to select them anyway, to AIR intrinsics that don't exist (such as air.atomic.global.store.i64). Now it leaves unsupported operations alone and reports them after the lowering as an InvalidIRError, with the same reasons and backtraces as validate_ir.

Compilations that validate are unaffected. The only case that changes is reflection on a kernel that couldn't be compiled anyway: it now shows why, instead of printing an intrinsic that doesn't exist. One of our own tests depended on that, with a 64-bit atomic store in the fence lowering test; it now uses a 32-bit store.

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.15%. Comparing base (082d689) to head (e6cbee3).

Files with missing lines	Patch %	Lines
src/metal.jl	90.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           tb/atomics     #943   +/-   ##
===========================================
  Coverage       87.15%   87.15%           
===========================================
  Files              30       30           
  Lines            6277     6285    +8     
===========================================
+ Hits             5471     5478    +7     
- Misses            806      807    +1     

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