Canonicalize an assume(load != null) into !nonnull metadata

Canonicalize an assume(load != null) into !nonnull metadata

We currently have two ways of informing the optimizer that the result of a load is never null: metadata and assume. This change converts the second in to the former. This avoids a need to implement optimizations using both forms.

We should probably extend this basic idea to metadata of other forms; in particular, range metadata. We view is that assumes should be considered a "last resort" for when there isn't a more canonical way to represent something.

Reviewed by: Hal
Differential Revision: http://reviews.llvm.org/D5951

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221737 91177308-0d34-0410-b5e6-96231b3b80d8