[LSR] Fix signed overflow in GenerateCrossUseConstantOffsets.

For the attached test case, unchecked addition of immediate starts and
ends overflows, as they can be arbitrary i64 constants.

Proof: https://rise4fun.com/Alive/Plqc

Reviewers: qcolombet, gilr, efriedma

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D59218

llvm-svn: 357217