[SCEV] Keep SCEVExpander insert points consistent.

Summary:
Make sure that the SCEVExpander Builder insert point and any
saved/restored insert points are kept consistent (i.e. their Instruction
and BasicBlock match) when moving instructions in SCEVExpander.

This fixes an issue triggered by
http://reviews.llvm.org/D18001 [LSR] Create fewer redundant instructions.

Test case will be added in reapply commit of above change:
http://reviews.llvm.org/D18480 Reapply [LSR] Create fewer redundant instructions.

Reviewers: sanjoy

Subscribers: mzolotukhin, sanjoy, qcolombet, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D20703

llvm-svn: 271424