[ConstantInt] Check active bits before calling getZExtValue.

Without this check, we hit an assertion in getZExtValue, if the constant
value does not fit into an uint64_t.

As getZExtValue returns an uint64_t, should we update
getAggregateElement to take an uin64_t as well?

This fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=6109.

Reviewers: efriedma, craig.topper, spatel

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D55547

llvm-svn: 348906