cmd/compile: suboptimal code for string equality

[randall77]

The optimizations of string equality are somewhat fragile. Particularly, the optimization of deciding when we don't need the full generality of memequal and can do something better instead.

The ordering of equality can make a difference, when it really shouldn't. Some orderings generate a memequal call, and some don't.

Compares that are obviously constant-sized, like those against the empty string, can sometimes still generate memequal calls.

Noticed as part of the truthy benchmarks, mentioned here.

[gopherbot]

Change https://go.dev/cl/485535 mentions this issue: cmd/compile: remove memequal call from string compares in more cases