[InstCombine] Preserve metadata when merging loads that are phi

arguments.

Make sure InstCombiner::FoldPHIArgLoadIntoPHI doesn't drop the following
metadata:

MD_tbaa
MD_alias_scope
MD_noalias
MD_invariant_load
MD_nonnull
MD_range

rdar://problem/17617709

Differential Revision: http://reviews.llvm.org/D12710

llvm-svn: 248419