x/tools/gopls: add a source action to see assembly for a range of lines

[firelizzard18]

Is your feature request related to a problem? Please describe.
I wanted to understand why this allocates:

func normalize(v any) (any, typeCode, bool) {
	switch v := v.(type) {
	case string:
		return v, typeCodeString, true
	// ...
	}
}

Describe the solution you'd like
I'd like to be able to select a line and view the assembly for that line (or for a range of lines).

Describe alternatives you've considered
I can (and did) use "Browse amd64 assembly for normalize" but that doesn't scale well to larger functions.

Additional context
#67478

[findleyr]

CC @adonovan

Since this is a gopls feature request, transferring to the Go issue tracker.

[findleyr]

Prior art: https://github.com/loov/lensm

[adonovan]

This is a reasonable request, though before we invest piecemeal in small improvements to the assembly UI, we should consider whether installing a more sophisticated disassembly visualization (including blocks and control-flow edges, similar to https://github.com/aclements/objbrowse or https://github.com/loov/lensm) is a better use of time.

As an immediate workaround, you can use the L%d line numbers in the third column to figure out which instructions belong to the source statement of interest.

[firelizzard18]

@adonovan I’d be happy with visualization like that. I’m fine changing this issue to that if you’d like.