[DAGCombiner] Remove overzealous assertion when folding assert+trunc+assert (PR55846)

These assert that there are no "useless" assertzext/assertsext nodes
(that assert a wider width than a following trunc), but I don't think
there is anything preventing such nodes from reaching this code.
I don't think the assertion is relevant for correctness of this
transform either -- if such an assert is present, then the other
one will always be to a smaller width, and we'll pick that one.
The assertion dates back to D37017.

Fixes https://github.com/llvm/llvm-project/issues/55846.

Differential Revision: https://reviews.llvm.org/D126952