[LSR] Check for signed overflow in NarrowSearchSpaceByDetectingSupersets.

We are adding a sign extended IR value to an int64_t, which can cause
signed overflows, as in the attached test case, where we have a formula
with BaseOffset = -1 and a constant with numeric_limits<int64_t>::min().

If the addition would overflow, skip the simplification for this
formula. Note that the target triple is required to trigger the failure.

Reviewers: qcolombet, gilr, kparzysz, efriedma

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D59211

llvm-svn: 356256