Fix comment in InstCombiner::visitAddrSpaceCast.

In the original version of the patch the behaviour was like described in
the comment.  This behaviour was changed before committing it without
updating the comment.
Roundtrip the inalloca bit on allocas through bitcode

This was an oversight in the original support.  As it is, I stuffed this
bit into the alignment.  The alignment is stored in log2 form, so it
doesn't need more than 5 bits, given that Value::MaximumAlignment is 1
<< 29.

Reviewers: nicholas

Differential Revision: http://reviews.llvm.org/D3943