[ConstantFold] Fix defect in constant folding computation for GEP

When the array indexes are all determined by GVN to be constants,
a call is made to constant-folding to optimize/simplify the address
computation.

The constant-folding, however, makes a mistake in that it sometimes reads
back stale Idxs instead of NewIdxs, that it re-computed in previous iteration.
This leads to incorrect addresses coming out of constant-folding to GEP.
A test case is included. The error is only triggered when indexes have particular
patterns that the stale/new index updates interplay matters.

Reviewers: Daniel Berlin
Differential Revision: https://reviews.llvm.org/D30642

llvm-svn: 297317