[Complexity] Optimize mctStereoIGF_dec_fx()

Basic info

This is a sub-task of issue #1009 (closed).

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 (closed)