refactor SLICE error_if checks

 * refactored error_if checks to avoid logic overlap
 * Added check to ensure that size dimensions match
output dimension
 * Defined the rank() function in pseudocode helpers

Change-Id: I56bf06252d9149d449c984126e8f2025a07d6034
Signed-off-by: Matthew Haddon <matthew.haddon@arm.com>