Replace _type with widenconst

[willtebbutt]

In my adventures to make everything in #426 work properly, I discovered that the compiler has a function Core.Compiler.widenconst, which is almost exactly what my Mooncake._type function does.

This PR looks at replacing Mooncake's _type functionalitywithwidenconst`, as this is obviously a better thing to do.

There's a slight discrepancy between how CC.widentconst and Mooncake._type handle CC.Conditionals, but I suspect my version is just wrong.

In this PR I've replaced the various methods of _type with a single method that just defers the call to CC.widenconst. This was the quickest way to make a PR and see if it works. If CI passes, I'll remove the _type function entirely, and replace all calls to _type with widenconst before merging.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
src/interpreter/abstract_interpretation.jl	83.33% <ø> (+0.28%)	⬆️
src/interpreter/ir_normalisation.jl	90.07% <100.00%> (ø)
src/interpreter/ir_utils.jl	87.50% <100.00%> (ø)
src/interpreter/s2s_reverse_mode_ad.jl	91.63% <100.00%> (+0.01%)	⬆️

[github-actions]

Performance Ratio:
Ratio of time to compute gradient and time to compute function.
Warning: results are very approximate! See here for more context.

┌────────────────────────────┬──────────┬─────────┬─────────────┬─────────┐
│                      Label │ Mooncake │  Zygote │ ReverseDiff │  Enzyme │
│                     String │   String │  String │      String │  String │
├────────────────────────────┼──────────┼─────────┼─────────────┼─────────┤
│                   sum_1000 │     70.0 │     1.0 │        5.61 │    8.21 │
│                  _sum_1000 │     6.69 │  1420.0 │        34.2 │    1.09 │
│               sum_sin_1000 │     2.26 │    1.68 │        10.7 │     2.0 │
│              _sum_sin_1000 │     2.75 │   247.0 │        13.8 │    2.51 │
│                   kron_sum │     80.4 │    10.7 │       309.0 │    10.2 │
│              kron_view_sum │     32.3 │    5.26 │       142.0 │    53.6 │
│      naive_map_sin_cos_exp │      2.5 │ missing │        7.15 │    2.33 │
│            map_sin_cos_exp │     2.74 │    1.52 │        6.13 │    2.91 │
│      broadcast_sin_cos_exp │     2.59 │    2.33 │        1.46 │    2.26 │
│                 simple_mlp │     5.06 │    3.11 │        7.49 │    3.48 │
│                     gp_lml │     13.3 │    6.69 │     missing │    6.56 │
│ turing_broadcast_benchmark │     3.31 │ missing │        24.9 │ missing │
│         large_single_block │     4.03 │  4340.0 │        42.7 │    2.18 │
└────────────────────────────┴──────────┴─────────┴─────────────┴─────────┘