Indices for getelementptr can be signed so we should use
getMinSignedBits instead of getActiveBits here. The function later calls
getSExtValue to get the int64_t value, which also checks
getMinSignedBits.
This fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=11647.
Reviewers: mssimpso, efriedma, davide
Reviewed By: efriedma
Differential Revision: https://reviews.llvm.org/D55536
llvm-svn: 348957
