[LV] Merge floating-point and integer induction widening code

This patch merges the existing floating-point induction variable widening code
into the integer induction variable widening code, creating a single set of
functions for both kinds of inductions. The primary motivation for doing this
is to enable vector phi node creation for floating-point induction variables.

Differential Revision: https://reviews.llvm.org/D30211

llvm-svn: 296145