Teach LazyValueInfo that allocas aren't NULL. Over all of llvm-test, this saves

half a million non-local queries, each of which would otherwise have triggered a
linear scan over a basic block.

Also fix a fixme for memory intrinsics which dereference pointers. With this,
we prove that a pointer is non-null because it was dereferenced by an intrinsic
112 times in llvm-test.

llvm-svn: 123533