Fix datatypes around opcodes.h and program.{c,h}

- Fix and clean-up datatypes in opcodes.h and program.{c,h}.
  Some of the changes were pointed by -Wconversion, others
  come from manual code inspection.

- Remove commented code from opcodes.h (unused for very long time).

- Add assertions to opcodes.h to check that instruction
  immediates (and other fields) don't exceed their range.

Change-Id: I7fc7c30ac2fdb00a93158d63d1379e7f16b1d168
Signed-off-by: Jan Matyas <[email protected]>

Author: JanMatCodasip