Metal: Drive the downgrader here, lower intrinsics during mcgen.

[maleadt]

Moves some magic out of Metal.jl, and also makes it possible to request both the LLVM IR as produced by Julia, as well as the AIR one with intrinsics lowered to AIR-specific ones. We may want to do this for PTX too (i.e., do the LLVM -> NVVM lowering late, during mcgen).

This is breaking for Metal.jl, but I'll retro-cap it when registering.
EDIT: turns out I won't have to because of the compat added here.

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.30%. Comparing base (5d88d3c) to head (af52be5).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/metal.jl	91.66%	1 Missing ⚠️
src/utils.jl	93.75%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #829   +/-   ##
=======================================
  Coverage   79.30%   79.30%           
=======================================
  Files          25       25           
  Lines        4517     4542   +25     
=======================================
+ Hits         3582     3602   +20     
- Misses        935      940    +5     

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