[LICM] Hoist assumes out of loops

If we have an assume which is known to execute and whose operand is invariant, we can lift that into the pre-header. So long as we don't change which paths the assume executes on, this is a legal transformation. It's likely to be a useful canonicalization as other transforms only look for dominating assumes.

Differential Revision: https://reviews.llvm.org/D50364

llvm-svn: 339481