Only demote unordered atomic loads and stores

[maleadt]

demote_atomics! (#904, #924, #927) turned every LLVM atomic load and store into a plain one on Metal and SPIR-V because Julia's GC orderings (unordered reference loads and stores, release type-tag stores) reached back-ends that cannot express them. This also silently weakened user atomics, such as UnsafeAtomics' load/store! and Atomix' get/set! on OpenCL, oneAPI, and POCL: the vendor compiler could hoist a spin-wait's atomic load.

#940 removes dead exception objects, and #949 rejects remaining allocations of objects that contain references, so the release type-tag stores no longer reach the back-ends. unordered loads of heap references still do when they don't involve an allocation, e.g. of a DataType field of an immutable struct, or of a reference to a mutable object, passed by reference to a @noinline function. Those fail SPIR-V validation with the Khronos translator, and Metal rejects them once it lowers LLVM atomics (#942), as they are pointer-sized and outside device and threadgroup memory. So this PR narrows the demotion to unordered loads and stores, which only promise not to tear, and leaves atomics with other orderings alone.

With the whole stack, test results are unchanged (see #949). The KernelAbstractions POCL test suite has 11 UnsafeAtomics/Atomix atomics that are no longer demoted.

Reflection (code_llvm, code_native) does not validate IR, so an object with references can still reach the back-end with its GC orderings, as can other invalid IR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.97%. Comparing base (e5adcf7) to head (971e6ae).

Additional details and impacted files

@@                         Coverage Diff                         @@
##           tb/reject-reference-allocations     #950      +/-   ##
===================================================================
- Coverage                            86.99%   86.97%   -0.03%     
===================================================================
  Files                                   29       29              
  Lines                                 5915     5904      -11     
===================================================================
- Hits                                  5146     5135      -11     
  Misses                                 769      769              

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