ptxed, block: extract fetching an instruction

Extract the code for fetching an instruction from a block into a separate
function.  We will need it in a few more places.

Change-Id: Ibe676184f37f8328a4b684ff7d13a6d38fe8aa82
Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>