[ConstantFold] Use getMinSignedBits for APInt in isIndexInRangeOfArrayType.

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