use SSE2 for is_valid_ascii

new thread [was: WIP Patch: Add a function that returns binary JSONB as a bytea]

> I wrote:
> > We can also shave a
> > few percent by having pg_utf8_verifystr use SSE2 for the ascii path. I
> > can look into this.
>
> Here's a patch for that. If the input is mostly ascii, I'd expect that
> part of the flame graph to shrink by 40-50% and give a small boost
> overall.

Here is an updated patch using the new USE_SSE2 symbol. The style is
different from the last one in that each stanza has platform-specific
code. I wanted to try it this way because is_valid_ascii() is already
written in SIMD-ish style using general purpose registers and bit
twiddling, so it seemed natural to see the two side-by-side. Sometimes
they can share the same comment. If we think this is bad for
readability, I can go back to one block each, but that way leads to
duplication of code and it's difficult to see what's different for
each platform, IMO.

--
John Naylor
EDB: http://www.enterprisedb.com