perf: Replace uses of genericLength with strictGenericLength

genericLength is a recursive function and marked NOINLINE. It is not
going to specialise. In profiles, it can be seen that 3% of total compilation
time when computing bytecode is spend calling this non-specialised
function.

In addition, we can simplify `addListToSS` to avoid traversing the input
list twice and also allocating an intermediate list (after the call to
reverse).

Overall these changes reduce the time spend in 'assembleBCOs' from 5.61s
to 3.88s. Allocations drop from 8GB to 5.3G.

Fixes #25706