[LV] Transform truncations of non-primary induction variables

The vectorizer tries to replace truncations of induction variables with new
induction variables having the smaller type. After r295063, this optimization
was applied to all integer induction variables, including non-primary ones.
When optimizing the truncation of a non-primary induction variable, we still
need to transform the new induction so that it has the correct start value.
This should fix PR32419.

Reference: https://bugs.llvm.org/show_bug.cgi?id=32419
llvm-svn: 298882