[ValueTracking] Check for ConstantExpr before using recursive helpers.

Make sure we do not call
constainsConstantExpression/containsUndefElement on ConstantExpression,
which is not supported.

In particular, containsUndefElement/constainsConstantExpression are only
supported on constants which are supported by getAggregateElement.

Unfortunately there's no convenient way to check if a constant supports
getAggregateElement, so just check for non-constantexpressions with
vector type. Other users of those functions do so too.

Reviewers: spatel, nikic, craig.topper, lebedev.ri, jdoerfert, aqjune

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D84512