Remove the demotion of 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. The GC orderings that required this workaround therefore no longer reach the back-ends, so this PR removes the demotion.

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 86.38%. Comparing base (89ef241) to head (78d617d).

Additional details and impacted files

@@                         Coverage Diff                         @@
##           tb/reject-reference-allocations     #950      +/-   ##
===================================================================
- Coverage                            86.39%   86.38%   -0.01%     
===================================================================
  Files                                   29       29              
  Lines                                 5805     5794      -11     
===================================================================
- Hits                                  5015     5005      -10     
+ Misses                                 790      789       -1     

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