[SCEV] Update interface to handle SCEVExpander insert point motion.

Summary:
This is an extension of the fix in r271424.  That fix dealt with builder
insert points being moved by SCEV expansion, but only for the lifetime
of the expand call.  This change modifies the interface so that LSR can
safely call expand multiple times at the same insert point and do the
right thing if one of the expansions decides to move the original insert
point.

This is a fix for PR28719.

Reviewers: sanjoy

Subscribers: llvm-commits, mcrosier, mzolotukhin

Differential Revision: https://reviews.llvm.org/D23342

llvm-svn: 278413