Improve hashtable prefetching

[chandlerc]

I had removed most but not all of the hashtable prefetching during development because I wasn't confident in the benchmarking results. However, I never revisited this once the benchmarking infrastructure improved and there were solid and stable results.

This factors the two interesting prefetch patterns I've seen for this style of hashtable into helpers that are always called, and provides macros that can be used during the build to configure exactly which prefetch strategies are enabled.

Benchmarking these and gaining confidence is very frustrating -- even now with the improved infrastructure, the noise is much higher than I would like. But it seems clear that some prefetching is a significant win. It also seems like enabling both results in too much prefetch traffic. And the entry group prefetch appears to be significantly more effective, both for the most interesting of the microbenchmarks and maybe most importantly for our compilation benchmarks. There, AMD is helped substantially and M1 seems to be helped some (although harder to measure).

AMD server benchmark numbers:

name                                              old cpu/op   new cpu/op   delta
BM_CompileAPIFileDenseDecls<Phase::Lex>/256       35.0µs ± 2%  34.2µs ± 2%  -2.40%  (p=0.000 n=20+19)
BM_CompileAPIFileDenseDecls<Phase::Lex>/1024       156µs ± 2%   151µs ± 2%  -3.18%  (p=0.000 n=20+19)
BM_CompileAPIFileDenseDecls<Phase::Lex>/4096       625µs ± 1%   605µs ± 1%  -3.22%  (p=0.000 n=19+18)
BM_CompileAPIFileDenseDecls<Phase::Lex>/16384     2.79ms ± 1%  2.69ms ± 2%  -3.67%  (p=0.000 n=17+19)
BM_CompileAPIFileDenseDecls<Phase::Lex>/65536     12.1ms ± 1%  11.6ms ± 1%  -4.30%  (p=0.000 n=17+18)
BM_CompileAPIFileDenseDecls<Phase::Lex>/262144    56.6ms ± 1%  53.8ms ± 1%  -5.00%  (p=0.000 n=18+17)
BM_CompileAPIFileDenseDecls<Phase::Parse>/256     61.1µs ± 2%  61.7µs ± 1%  +0.87%  (p=0.000 n=19+19)
BM_CompileAPIFileDenseDecls<Phase::Parse>/1024     288µs ± 1%   290µs ± 1%  +0.55%  (p=0.004 n=20+20)
BM_CompileAPIFileDenseDecls<Phase::Parse>/4096    1.16ms ± 1%  1.16ms ± 1%  -0.54%  (p=0.000 n=17+19)
BM_CompileAPIFileDenseDecls<Phase::Parse>/16384   4.98ms ± 1%  4.91ms ± 1%  -1.39%  (p=0.000 n=20+19)
BM_CompileAPIFileDenseDecls<Phase::Parse>/65536   20.9ms ± 1%  20.5ms ± 1%  -1.86%  (p=0.000 n=20+19)
BM_CompileAPIFileDenseDecls<Phase::Parse>/262144  92.1ms ± 1%  90.2ms ± 1%  -2.12%  (p=0.000 n=18+20)
BM_CompileAPIFileDenseDecls<Phase::Check>/256     1.16ms ± 2%  1.16ms ± 1%    ~     (p=0.931 n=19+19)
BM_CompileAPIFileDenseDecls<Phase::Check>/1024    2.17ms ± 2%  2.16ms ± 1%    ~     (p=0.247 n=20+19)
BM_CompileAPIFileDenseDecls<Phase::Check>/4096    6.07ms ± 1%  6.04ms ± 1%  -0.48%  (p=0.007 n=19+19)
BM_CompileAPIFileDenseDecls<Phase::Check>/16384   22.4ms ± 1%  22.2ms ± 1%  -0.99%  (p=0.000 n=20+19)
BM_CompileAPIFileDenseDecls<Phase::Check>/65536   93.3ms ± 1%  92.2ms ± 1%  -1.23%  (p=0.000 n=20+18)
BM_CompileAPIFileDenseDecls<Phase::Check>/262144   400ms ± 1%   391ms ± 1%  -2.15%  (p=0.000 n=20+18)