[SeparateConstOffsetFromGEP] sext(a)+sext(b) => sext(a+b) when a+b can't sign-overflow.

Summary:
This patch implements my promised optimization to reunites certain sexts from
operands after we extract the constant offset. See the header comment of
reuniteExts for its motivation.

One key building block that enables this optimization is Bjarke's poison value
analysis (D11212). That helps to prove "a +nsw b" can't overflow.

Reviewers: broune

Subscribers: jholewinski, sanjoy, llvm-commits

Differential Revision: http://reviews.llvm.org/D12016

llvm-svn: 245003