[InstCombine] foldConstantInsEltIntoShuffle - bail if we fail to find constant element (PR51824)

If getAggregateElement() returns null for any element, early out as otherwise we will assert when creating a new constant vector

Fixes PR51824 + ; OSS-Fuzz: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=38057