x/tools/go/analysis/passes/unusedwrite: detect dead stores to local vars

[adonovan]

During review of https://go.dev/cl/581555, @timothy-king pointed out that I had defined a local variable using an if/else statement and then failed to use it after the statement, something like this:

 func f(pass *Pass), filename string) {
 	readFile := pass.ReadFile
 	if readFile == nil {
 		readFile = os.ReadFile
 	}
 	os.ReadFile(filename) // oops, I meant readFile
 }

The assignment to readFile is a dead store. We should flag this is a likely error.