[LICM] sink through non-trivially replicable PHI

Summary:
The current LICM allows sinking an instruction only when it is exposed to exit
blocks through a trivially replacable PHI of which all incoming values are the
same instruction. This change enhance LICM to sink a sinkable instruction
through non-trivially replacable PHIs by spliting predecessors of loop
exits.

Reviewers: hfinkel, majnemer, davidxl, bmakam, mcrosier, danielcdh, efriedma, jtony

Reviewed By: efriedma

Subscribers: nemanjai, dberlin, llvm-commits

Differential Revision: https://reviews.llvm.org/D37163

llvm-svn: 317335