[LoopVectorize][NFC] Refactor code to use IRBuilder::CreateStepVector

In places where we create a ConstantVector whose elements are a
linear sequence of the form <start, start + 1, start + 2, ...>
I've changed the code to make use of CreateStepVector, which creates
a vector with the sequence <0, 1, 2, ...>, and a vector addition
operation. This patch is a non-functional change, since the output
from the vectoriser remains unchanged for fixed length vectors and
there are existing asserts that still fire when attempting to use
scalable vectors for vectorising induction variables.

In a later patch we will enable support for scalable vectors
in InnerLoopVectorizer::getStepVector(), which relies upon the new
stepvector intrinsic in IRBuilder::CreateStepVector.

Differential Revision: https://reviews.llvm.org/D97861