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

Noticed on D159533 and I've finally deal 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.