[LV] Keep Primary Induction alive when folding tail by masking

Fix PR47390.

The primary induction should be considered alive when folding tail by masking,
because it will be used by said masking; even when it may otherwise appear
useless: feeding only its own 'bump', which is correctly considered dead, and
as the 'bump' of another induction variable, which may wrongfully want to
consider its bump = the primary induction, dead.

Differential Revision: https://reviews.llvm.org/D92017