reland: [DAG] Fix PR45049: LegalizeTypes crash

Sometimes LegalizeTypes knows about common subexpressions before SelectionDAG
does, leading to accidental SDValue removal before its reference count was
truly zero.

Differential Revision: https://reviews.llvm.org/D76994

Reviewed-By: bjope

Fixes: https://bugs.llvm.org/show_bug.cgi?id=45049

Reverted in 3ce77142 because the previous patch
broke the expensive-checks bots. The new patch removes the broken check.