intel/compiler: extract subfunctions of lower_integer_multiplication()

intel/compiler: extract subfunctions of lower_integer_multiplication()

The lower_integer_multiplication() function is already a little too
big. I want to add more to it, so let's reorganize the existing code
first. Let's start with just extracting the current code to
subfunctions. Later we'll change them a little more.

v2: Make private functions private (Caio).
v3: Fix typo (Caio).

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>