[LICM] Allow sinking when foldable in loop

Summary:
Continue trying to sink an instruction if its users in the loop is foldable.
This will allow the instruction to be folded in the loop by decoupling it from
the user outside of the loop.

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

Reviewed By: hfinkel

Subscribers: javed.absar, bmakam, mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D37076

llvm-svn: 320823