Completely rewrite how getelementptr instructions are expanded. This has two

(minor) benefits right now:

1. An extra dummy MOVrr32 is gone.  This move would often be coallesced by
   both allocators anyway.
2. The code now uses the gep_type_iterator to walk the gep, which should future
   proof it a bit.  It still assumes that array indexes are Longs though.

These don't really justify rewriting the code.  The big benefit will come later
though.

llvm-svn: 11710