strings: assembly optimization causes regression in functionality

[dsnet]

Commit 6b02a19 caused a regression in the strings package that causes some tests to fail.

A simple test to exercise this behavior is the following:

strings.Contains("abcdef?v=profileM", "abcdef?v=profile&")

On go1.6, this returns false as expected.
On 6b02a19, this returns true, which is incorrect.

/cc @rsc, @TocarIP, @nigeltao

[bradfitz]

/cc @randall77 too

[gopherbot]

CL https://golang.org/cl/23106 mentions this issue.

[minux]

Should this issue remain open to track the bugfix for Go 1.8?

[randall77]

I'll open a separate issue.