cmd/compile: generate better eq algs for some array types

[josharian]

func geneq in alg.go contains this comment:

	case TARRAY:
		// An array of pure memory would be handled by the
		// standard memequal, so the element type must not be
		// pure memory. Even if we unrolled the range loop,
		// each iteration would be a function call, so don't bother
		// unrolling.

This is not quite true. Floats can’t be compared as pure mem, but don’t require a function call.

And some things that do require a function call are nevertheless susceptible to more optimization. For example, for comparing arrays of strings, we could compare all lengths first (possibly unrolled), and only compare bytes if all lengths agree. Probably something similar for interfaces.

We might even want to inline or partially inline some of these cases.

[cagedmantis]

/cc @randall77 @griesemer

[josharian]

I did a bunch of this for 1.15. Going to call this done enough. The remaining outstanding cases (arrays of structs, arrays of arrays) are rare and already have a separate issue filed.