asix-sigma: rephrase trigger LUT creation (mechanical change)

asix-sigma: rephrase trigger LUT creation (mechanical change)

Rephrase the sigma_build_basic_trigger() and build_lut_entry() routines
to hopefully improve readability. Avoid the use of short and generic
identifiers which are just too easy to confuse with each other and the
1 literal and negation operator in deeply nested loops and complex
expressions that span several text lines. Reduce indentation where
appropriate. Concentrate initialization and use of variables such that
reviewers need less context for verification.

This is a purely mechanical change, the function of triggers remains
untested for now. Setting "selres" in that spot is suspicious, too.