The old implementation assumed that all memory runs
were terminated by non-memory fields.
This isn't necessarily so.
They might be terminated by padding or blank fields.

For example, given

type T struct {
	a int64
	b byte
	c, d, e int64
}

the old implementation did a memory comparison on a+b, on c, and on d+e.

Instead, check for memory runs at the beginning of every round.
This now generates a memory comparison on a+b and on c+d+e.

Also, delete some now-dead code.

Change-Id: I66bffb111420adf6919bd708e4fb3a1e1f07fadd
Reviewed-on: https://go-review.googlesource.com/19841Reviewed-by: David Crawshaw <crawshaw@golang.org>