strings: fix indexShortStr for 17+ bytes

[randall77]

There's a bug in indexShortStr for 17+ bytes. That case has been disabled for the 1.7 release. This bug is to track the fix and reenabling of this case for 1.8.
See issue #15679 .

@TocarIP

[randall77]

I think maybe the fix is simple. The code mistakenly uses the 16-byte code when there are 17 bytes.

_16_or_more:
    CMPQ AX, $17
    JA   _17_to_31

The fix may be a simple as using JAE instead.
But I want some more tests before reenabling.

[minux]

I think we also need much more extensive tests on those newly introduced (optimized) assembly functions before Go 1.7. Recent experiences showed that they are not as reliable as one would expect (e.g. #15617 for crypto/sha1)

[TocarIP]

This is caused by

-       CMPQ BX, $16
+       CMPQ AX, $17

from 6b02a19

Changing 17 back to 16 indeed works.

[gopherbot]

CL https://golang.org/cl/23440 mentions this issue.