Re-commit : [LICM] Allow sinking when foldable in loop

This recommits r320823 reverted due to the test failure in sink-foldable.ll and
an unused variable. Added "REQUIRES: aarch64-registered-target" in the test
and removed unused variable.

Original commit message:

  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: 320858