Ignore/Drop droppable uses for code-sinking in InstCombine

Summary:
This patch allows code-sinking in InstCombine to be performed when instruction have uses in llvm.assume.

Use are considered droppable when it is preferable to modify the User such that the use disappears rather than to prevent a transformation because of the use.
for now uses are considered droppable if they are in an llvm.assume.

Reviewers: jdoerfert, nikic, spatel, lebedev.ri, sstefan1

Reviewed By: jdoerfert

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73832