[Patch] Speed up operator>>(istream, string)

[Paolo Carlini]

Hi,

the below consistently improves operator>>(..., string) in the same way of
getline(..., string).

For a testcase identical to streams.cpp in 15002, besides 'f >> l' instead
of 'getline(f, l)', these are some numbers on my usual machine (on x86_64
the results are similar):

3.4.0
-----
9.090u 0.080s 0:09.22 99.4%     0+0k 0+0io 206pf+0w

3.4.1 pre
---------
3.470u 0.140s 0:03.81 94.7%     0+0k 0+0io 204pf+0w

3.4.1 pre + patch
-----------------
1.400u 0.110s 0:01.51 100.0%    0+0k 0+0io 205pf+0w 

3.5 exp
-------
7.050u 0.090s 0:07.18 99.4%     0+0k 0+0io 208pf+0w

3.5 exp + patch
---------------
1.500u 0.080s 0:01.58 100.0%    0+0k 0+0io 209pf+0w

Icc8.0
------
2.770u 0.110s 0:02.89 99.6%     0+0k 0+0io 203pf+0w


As you can see, the code generated by unpatched mainline is not as good
as 3_4-branch: sadly, I'm seeing this rather often, for other performance
testcases in the iostreams area too.

Anyway, of course the patch would go in mainline only, for now, and I'm
also adding a couple of new testcases.

A will wait a few hours, in case of comments.

Regtested x86-linux.

Paolo.

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