[DAG] getNode() - remove oneuse limit from (zext (trunc (assertzext x))) ->...

[DAG] getNode() - remove oneuse limit from (zext (trunc (assertzext x))) -> (assertzext x) fold (REAPPLIED)

Noticed on D159533 and I've finally dealt with the x86 regressions - MatchingStackOffset wasn't peeking through AssertZext nodes while trying to find CopyFromReg/Load sources, it was only removing them if they were part of a (trunc (assertzext x)) pattern.

Reapplied after being reverted at 4389252c - which should be addressed by D159537 / 6d267999