Drop thrown values on targets that can't throw

[maleadt]

Targets that cannot throw (!can_throw(job)) report exceptions without inspecting the thrown value, but code still constructed that value. The construction is dead after throw is lowered, yet often survived to the back-end because Julia does not inline into throw blocks and the constructors allocate through the device allocator. For example, Bool(x) constructs an InexactError whose boxed args tuple gets an unordered pointer store (#904), while log1p with Float64 emulation constructs a DomainError with a LazyString (release stores). These GC orderings led to demote_atomics! (#904, #924, #927).

This PR adds drop_throw_arguments!, a Julia IR pass for targets where !can_throw(job). It rewrites throw(x) as throw(nothing). Julia's DCE (compact! + adce_pass!) then removes each construction proven removable_if_unused; constructions with side effects, such as eagerly built message strings, remain. The pass replaces lower_throw!'s LLVM-level erasure of the thrown value, which erased the call's direct argument regardless of what it was.

Measurements

On Julia 1.12 on an M1, these are the numbers of kernels that passed validation but still contained dead exception allocations, GC atomics, or boxes. The measurements use the whole stack; later PRs in the stack do not remove any of the constructs counted here:

• Metal Base-function sweep: 37 → 0.
• Same sweep with Float64 emulation (Metal.jl#955 + Software Float64 emulation, and job-scoped device libraries #926): 61 → 0.
• oneAPI Base sweep, compile-only: 108 → 0.
• KernelAbstractions' POCL test suite: gpu_convert_kernel! had 8 InexactError allocations and 24 store atomic ptr unordered → 0.
• OpenCL PoCL tests: 3 DomainError float boxes → 0.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.28%. Comparing base (1ad3695) to head (946c1d7).

Additional details and impacted files

@@                  Coverage Diff                   @@
##           tb/julia-ir-passes     #940      +/-   ##
======================================================
- Coverage               86.36%   86.28%   -0.09%     
======================================================
  Files                      29       29              
  Lines                    5766     5783      +17     
======================================================
+ Hits                     4980     4990      +10     
- Misses                    786      793       +7     

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[maleadt]

Looks to be working nicely! And allows us to get rid of the broken atomic demotion pass.