Import Vellvm change from simplberry.

[jeehoonkang]

• This patch is modifications accumulated during simplberry development.
• Notable changes are:
  • Fix Axiom name mismatches. https://github.com/snu-sf/vellvm-legacy/compare/vellvm:master...master#diff-661cba919972f25b608da82577c358a6L1126
  • Memory allocation does not check if the size is not positive. First, at least, malloc(0) should works well. Second, Mem.alloc already has check routine.
  • Memory store performs dynamic type check, to match with memory load.
• This commit is not production-ready: containing a lot of comments, etc. Let's discuss on the change, and clean the code and merge.

[dgarbuzov]

We're discussing the issues with extraction in another thread, and I think we talked about adding the tag checking to stores on Skype. That and the update to malloc I think are correct w.r.t the semantics of LLVM/C, respectively.

The apparent typo in callFree__extcall_properties is troubling. I'm still not sure I understand external_intrinsics.v -- why does a call to malloc/free result in an empty trace?

Nitpicking:

• it would be nice to put the extracted files in a separate directory. The only way I know to do this is to call Cd "./otherdir" in the extraction script.
• Removing commented code prior to committing would make the diffs slightly easier to read.