x86_64: begin rewriting instruction selection

[jacobly0]

Closes #21950

[jacks0n9]

comically large pr

[andrewrk]

Wow, congrats!

[andrewrk]

Incredible. I didn't realize just how much went into this branch until now when I reviewed it (locally of course since GitHub can't be bothered to display the diff).

The cg.select abstraction is pretty nifty.

Happy to see this land in master branch.

[andrewrk]

perf data point: building hello world

Benchmark 1 (24 runs): stage3-0.14.0-dev.2802+257054a14/bin/zig build-exe ../test/standalone/simple/hello_world/hello.zig -fno-llvm -fno-lld
  measurement          mean ± σ            min … max           outliers         delta
  wall_time           213ms ± 10.0ms     200ms …  239ms          2 ( 8%)        0%
  peak_rss            119MB ±  632KB     118MB …  120MB          0 ( 0%)        0%
  cpu_cycles         1.15G  ± 10.6M     1.14G  … 1.19G           6 (25%)        0%
  instructions       2.38G  ± 76.0K     2.38G  … 2.38G           1 ( 4%)        0%
  cache_references   75.7M  ±  740K     75.0M  … 78.3M           1 ( 4%)        0%
  cache_misses       5.80M  ± 53.9K     5.70M  … 5.94M           1 ( 4%)        0%
  branch_misses      6.82M  ± 48.3K     6.76M  … 6.95M           2 ( 8%)        0%
Benchmark 2 (25 runs): stage3-0.14.0-dev.2828+4bace0f62/bin/zig build-exe ../test/standalone/simple/hello_world/hello.zig -fno-llvm -fno-lld
  measurement          mean ± σ            min … max           outliers         delta
  wall_time           201ms ± 7.76ms     184ms …  218ms          0 ( 0%)        ⚡-  5.6% ±  2.4%
  peak_rss            120MB ±  878KB     118MB …  122MB          3 (12%)          +  0.7% ±  0.4%
  cpu_cycles         1.10G  ± 7.16M     1.09G  … 1.12G           0 ( 0%)        ⚡-  3.9% ±  0.5%
  instructions       2.24G  ±  635K     2.24G  … 2.24G           3 (12%)        ⚡-  6.0% ±  0.0%
  cache_references   80.1M  ±  220K     79.7M  … 80.5M           0 ( 0%)        💩+  5.7% ±  0.4%
  cache_misses       6.80M  ± 77.5K     6.67M  … 6.93M           0 ( 0%)        💩+ 17.3% ±  0.7%
  branch_misses      6.79M  ± 25.4K     6.74M  … 6.87M           0 ( 0%)          -  0.4% ±  0.3%