1. LiveIntervals now implement a 4 slot per instruction model. Load,
   Use, Def and a Store slot. This is required in order to correctly
   represent caller saved register clobbering on function calls,
   register reuse in the same instruction (def resues last use) and
   also spill code added later by the allocator. The previous
   representation (2 slots per instruction) was insufficient and as a
   result was causing subtle bugs.

2. Fixes in spill code generation. This was the major cause of
   failures in the test suite.

3. Linear scan now has core support for folding memory operands. This
   is untested and not enabled (the live interval update function does
   not attempt to fold loads/stores in instructions).

4. Lots of improvements in the debugging output of both live intervals
   and linear scan. Give it a try... it is beautiful :-)

In summary the above fixes all the issues with the recent reserved
register elimination changes and get the allocator very close to the
next big step: folding memory operands.

llvm-svn: 11654