[SCEVExpander] Have hoistIVInc preserve LCSSA

Summary:
(Note: the problematic invocation of hoistIVInc that caused PR24804 came
from IndVarSimplify, not from SCEVExpander itself)

Fixes PR24804.  Test case by David Majnemer.

Reviewers: hfinkel, majnemer, atrick, mzolotukhin

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D15058

llvm-svn: 254976