[IndVars] Preserve LCSSA in `eliminateIdentitySCEV`

Summary:
After r249211, SCEV can see through some LCSSA phis.  Add a
`replacementPreservesLCSSAForm` check before replacing uses of these phi
nodes with a simplified use of the induction variable to avoid breaking
LCSSA.

Fixes 25047.

Depends on D13460.

Reviewers: atrick, hfinkel

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D13461

llvm-svn: 249575