repeat() function, CHECK_FOR_INTERRUPTS(), and unlikely()

I was doing some memory testing under fractional CPU allocations and it became
painfully obvious that the repeat() function needs CHECK_FOR_INTERRUPTS().

I exchanged a few emails offlist with Tom about it, and (at the risk of putting
words in his mouth) he agreed and felt it was a candidate for backpatching.

Very small patch attached. Quick and dirty performance test:

explain analyze SELECT repeat('A', 300000000);
explain analyze SELECT repeat('A', 300000000);
explain analyze SELECT repeat('A', 300000000);

With an -O2 optimized build:

Without CHECK_FOR_INTERRUPTS

Planning Time: 1077.238 ms
Execution Time: 0.016 ms

Planning Time: 1080.381 ms
Execution Time: 0.013 ms

Planning Time: 1072.049 ms
Execution Time: 0.013 ms

With CHECK_FOR_INTERRUPTS

Planning Time: 1078.703 ms
Execution Time: 0.013 ms

Planning Time: 1077.495 ms
Execution Time: 0.013 ms

Planning Time: 1076.793 ms
Execution Time: 0.013 ms

While discussing the above, Tom also wondered whether we should add unlikely()
to the CHECK_FOR_INTERRUPTS() macro.

Small patch for that also attached. I was not sure about the WIN32 stanza on
that (to do it or not; if so, what about the UNBLOCKED_SIGNAL_QUEUE() test).

I tested as above with unlikely() and did not see any discernible difference,
but the added check might improve other code paths.

Comments or objections?

Thanks,

Joe
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