Resolve small type tags when looking up referenced objects

[maleadt]

Fixes the crash reported on #949; meant to be folded into that PR.

Since Julia 1.10, codegen refers to some types by a small tag rather than their address: an offset into jl_small_typeof, below jl_max_tags << 4. On 1.12+, Core.svec is allocated inline with such a tag as the type operand:

call ptr @julia.gc_alloc_obj(ptr %current_task, i64 16, ptr inttoptr (i64 144 to ptr))

check_allocation! passes that operand to referenced_object, which treated every inttoptr constant as an object address, so compiling a kernel like this segfaulted:

using Metal
k(a) = (Base.donotdelete(Core.svec(a[1])); nothing)
Metal.code_llvm(devnull, k, Tuple{MtlDeviceVector{Float32,1}}; kernel = true)

referenced_object now looks up constants below jl_max_tags << 4 in jl_small_typeof, as jl_to_typeof does. Unused table entries and non-constant inttoptr operands are treated as unknown. Larger constants are still treated as object addresses, as before: 1.10's codegen and :bake relocations embed them. The svec allocation above is now reported as unsupported allocation of an object with references (Core.SimpleVector).

Tests: the Metal unsupported allocations testset has a kernel that allocates an svec, checked on 1.12+ because older versions call jl_f_svec instead. It also calls referenced_object directly, on every version, for a small tag, an unused tag and a type address. Without the fix, the test worker segfaults. I ran the Metal tests on 1.10 through 1.13, the SPIR-V tests on the same versions, and the full suite on 1.10 and 1.12. PTX and GCN tests were skipped because those back-ends aren't available on macOS. With Metal.jl 1.11.1, the reproducer above no longer crashes, and launching the kernel with @metal raises the InvalidIRError.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.56%. Comparing base (89ef241) to head (c019afb).

Additional details and impacted files

@@                         Coverage Diff                         @@
##           tb/reject-reference-allocations     #961      +/-   ##
===================================================================
+ Coverage                            86.39%   86.56%   +0.17%     
===================================================================
  Files                                   29       29              
  Lines                                 5805     5813       +8     
===================================================================
+ Hits                                  5015     5032      +17     
+ Misses                                 790      781       -9     

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