cmd/compile: binaries contain many ..type.eq funcs

[bradfitz]

We have a meta bug for binary size (#6853), but as one specific item:

Go binaries contain many ..type.eq funcs to support == on types at runtime.

The compiler & linker could probably omit provably unneeded ones. Or even do some with slower reflect if they're large & unlikely to be needed at runtime.

Background:

• https://twitter.com/bradfitz/status/1255704982893912064,
• https://go-review.googlesource.com/c/go/+/231118 (http)
• https://go-review.googlesource.com/c/net/+/231119 (http2)
• https://go-review.googlesource.com/c/go/+/231239 (runtime)

[gopherbot]

Change https://golang.org/cl/231119 mentions this issue: http2: mark some structs as non-comparable

[randall77]

In https://go-review.googlesource.com/c/go/+/191198 I did a similar thing for hash functions. Only the ones we know are needed are generated. The rest are done reflect-like inside the runtime.
The slowdown wasn't too bad (20%) and it saved a lot of binary size (1%). The nice thing about maps is we know most of the hash functions we need, as interface-keyed maps are the only exception and they are fairly rare (rarer still, non-pointer interface keys). It's less clear for == because those comparison functions are used in more places.

[gopherbot]

Change https://golang.org/cl/231118 mentions this issue: net/http: remove badStringError, make some unexported structs non-comparable

[gopherbot]

Change https://golang.org/cl/231397 mentions this issue: cmd/link: don't mark a symbol's Gotype reachable

[martisch]

I saw some [x]interface{} algs in @bradfitz tweet which may be created as part of variadic functions.
We might have a few places left were we create algs for compiler generated arrays. I should do a pass to find out and then close the issue: #28639

[randall77]

I tried to noalg slice backing store arrays, but trybots didn't like me:
https://go-review.googlesource.com/c/go/+/151497
Never got around to figuring out why.

[cuonglm]

I tried to noalg slice backing store arrays, but trybots didn't like me:
https://go-review.googlesource.com/c/go/+/151497
Never got around to figuring out why.

I think the reason is the same as 343b7fa