Change a stupid algorithm that deals with real register live

Change a stupid algorithm that deals with real register live
ranges into a less stupid one.  Prior to this change, the complexity
of reg-alloc included an expensive term

O(#instrs in code sequence x #real-register live ranges in code sequence)

This commit changes that term to essentially

O(#instrs in code sequence) + O(time to sort real-reg-L-R array)

On amd64 this nearly halves the cost of register allocation and means
Valgrind performs better in translation-intensive situations (a.k.a
starting programs).  Eg, firefox start/exit falls from 119 to 113
seconds.  The effect will be larger on ppc32/64 as there are more real
registers and hence real-reg live ranges to consider, and will be
smaller on x86 for the same reason.

The actual code the JIT produces should be unchanged.  This commit
merely modifies how the register allocator handles one of its
important data structures.

git-svn-id: svn://svn.valgrind.org/vex/trunk@1686