[LV] Ensure fixNonInductionPHIs uses a valid insertion point.

In some cases, Builder's insertion point may be invalidated before using
it in VPTransformState::get. Make sure the insertion point is
up-to-date.

This should fix various sanitizer errors, like
https://lab.llvm.org/buildbot/#/builders/5/builds/4933/steps/9/logs/stdio