Several changes:

  * Simplify handling of byval, making it easier to understand and more 
    consistent.  This fixes PR2065.
  * Clean up and simplify handling of GEPs.  I can actually understand it now!
  * Implement support for GEP'ing into vectors, this fixes
    SingleSource/UnitTests/Vector/build2 among others.

llvm-svn: 47818