[IR] Change CreateStepVector to work with element types smaller than i8

Currently the stepvector intrinsic only supports element types that
are integers of size 8 bits or more. This patch adds support for the
creation of stepvectors with smaller element types by creating
the intrinsic with i8 elements that we then truncate to the requested
size.

It's not currently possible to write a vectoriser test to exercise
this code path so I have added a unit test here:

  llvm/unittests/IR/IRBuilderTest.cpp

Differential Revision: https://reviews.llvm.org/D113767