• lo-mappers.h: add two "fits-into-int" predicates, one symmetric

  [INT_MIN+1, INT_MAX], the other "regular" [INT_MIN, INT_MAX].

• *xpow.cc, op-s*m-s.cc: use them where appropriate.

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As discussed in
https://mailman.cae.wisc.edu/pipermail/octave-maintainers/2013-November/037580.html

Notes:

- Cases where xisint() was used were not touched as they are already correct (but may be a little too strict). This is for ease of review, plus my understanding is that the interpreter has now to be kept in sync with the JIT compiler. Thus, no well-defined behavior should have been changed.
- I only tested an Octave-default build without the JIT compiler, as that one gave build errors for an LLVM from SVN (is Octave following a particular LLVM build, such as one available with a distro's package management?)
- The predicates use ceil() as that one is declared inline (see below)

Additional attachments:

- xpow_tests.m: an annotated M script containing Sanitizer output before the cleanup and some other annotations
- xisint.cc: a very simple C++ test file for timing the predicates when run in a loop. Results: inlining seems to give a noticeable speedup (hence ceil()), my versions compile to somewhat shorter code.

xpow_test.m also shows something curious: running
 a = -single([1,2;3,4]); b=single([1 1; 1 2^32]); a.^b
calls the
 (const FloatNDArray& a, const FloatNDArray& b)
overload, not the FloatMatrix one. Not a big deal I guess, since the former is more general, but maybe noteworthy.