3x performance regression

[mratsim]

I detected a huge performance regression in Arraymancer on my benchmark: https://github.com/mratsim/Arraymancer/blob/master/benchmarks/ex01_xor.nim

It used to run in 10sec but now takes about 30 seconds to run (see mratsim/Arraymancer#221 for screenshots).

I've narrowed down the issue to this commit: 358709e, the previous commit worked OK: 2c1f1f2.

I have several {.noInit.} in the library to avoid initialization where not needed but it seems like that is now completely ignored.

[mratsim]

I confirm the fix. Actually now devel is about 30% faster than before, some perf improvement were hidden by this regression 🎆 .