[analyzer] LoopUnrolling: check the bitwidth of the used numbers (pr34943)

The loop unrolling feature aims to track the maximum possible steps a loop can
make. In order to implement this, it investigates the initial value of the 
counter variable and the bound number. (It has to be known.)
These numbers are used as llvm::APInts, however, it was not checked if their
bitwidths are the same which lead to some crashes.
This revision solves this problem by extending the "shorter" one (to the length
of the "longer" one).
For the detailed bug report, see: https://bugs.llvm.org/show_bug.cgi?id=34943

Differential Revision: https://reviews.llvm.org/D38922

llvm-svn: 316830