refactor perform_lowlevel_parse for readability

Drive-by elimination of an age-old TODO. :)
Code should be functionally equivalent. Changes:

 - Remove broken code under CONSISTENCY_CHECK.
 - Transform to chip-away style.
 - Remove braces around single statements.
 - Declare variables at top of function.
 - Shorten some variable names (tmp_res and bit_length).
 - Limit line length to 80 columns.