Correctly combine alias.scope metadata by a union instead of intersecting

Summary:
The alias.scope metadata represents sets of things an instruction might
alias with. When generically combining the metadata from two
instructions the result must be the union of the original sets, because
the new instruction might alias with anything any of the original
instructions aliased with.

Reviewers: hfinkel

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D7490

llvm-svn: 228525