…, C1). NFC

Add test where the zext has an additional use, but the entire
expression can be replaced with (zext X). Folding even though there
is an additional use would not increase the number of instructions.

…dditional use.

We have a general fold for (zext (X +nuw C2)) + C1 --> zext (X + (C2 + trunc(C1)))
but this fold is disabled if the zext has an additional use.

If the two constants cancel, we can fold the whole expression to
zext(X) without increasing the number of instructions.

Though we will have 2 zexts which might be not be cheap. So
another option could be to move the original narrow add after the zext
for the other uses. That would allow sharing of the zext(x).