Enable ISL's small integer optimization

Summary:
With small integer optimization (short: sio) enabled, ISL uses 32 bit
integers for its arithmetic and only falls back to a big integer library
(in the case of Polly: IMath) if an operation's result is too large.
This gives a massive performance boost for most application using ISL.
For instance, experiments with ppcg (polyhedral source-to-source
compiler) show speed-ups of 5.8 (compared to plain IMath), respectively
2.7 (compared to GMP).

In Polly, a smaller fraction of the total compile time is taken by ISL,
but the speed-ups are still very significant. The buildbots measure
compilation speed-up up to 1.8 (oourafft, floyd-warshall, symm). All
Polybench benchmarks compile in at least 9% less time, and about 20%
less on average.

Detailed Polybench compile time results (median of 10):
correlation     -25.51%
covariance      -24.82%
2mm             -26.64%
3mm             -28.69%
atax            -13.70%
bicg            -10.78%
cholesky        -40.67%
doitgen         -11.60%
gemm            -11.54%
gemver          -10.63%
gesummv         -11.54%
mvt              -9.43%
symm            -41.25%
syr2k           -14.71%
syrk            -14.52%
trisolv         -17.65%
trmm             -9.78%
durbin          -19.32%
dynprog          -9.09%
gramschmidt     -15.38%
lu              -21.77%
floyd-warshall  -42.71%
reg_detect      -41.17%
adi             -36.69%
fdtd-2d         -32.61%
fdtd-apml       -21.90%
jacobi-1d-imper  -9.41%
jacobi-2d-imper -27.65%
seidel-2d       -31.00%

Reviewers: grosser

Reviewed By: grosser

Subscribers: Meinersbur, llvm-commits, pollydev

Projects: #polly

Differential Revision: http://reviews.llvm.org/D10506

llvm-svn: 240689

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