[StaticAnalyzer] LoopUnrolling: Keep track the maximum number of steps for each loop

This way the unrolling can be restricted for loops which will take at most a
given number of steps. It is defined as 128 in this patch and it seems to have
a good number for that purpose.

Differential Revision: https://reviews.llvm.org/D37181

llvm-svn: 311883