[Complexity] Optimize mctStereoIGF_dec_fx()
# Basic info
This is a sub-task of issue #1009.
# Bug description
Various optimizations to mctStereoIGF_dec_fx():
* Replace costly divisions by shr - we only need to divide by 1 or 2
* Calculate constant value for shift outside a for loop
# Ways to reproduce
See commandlines in issue #1009
issue