IslCodegen: Do not build upper bound in vector for

For for-nodes that are translated to a set of vector lanes, we already know the
overall number of iterations. Calculating the upper bound is consequently not
necessary. This change removes the code for upper bound calculation, which was
probably copy/pasted from the code generation for the normal for-loop.

This issue was found by Sylvestre's scan-build server.

llvm-svn: 193925