want to insert a new range into the middle of the vector, then delete ranges
one at a time next to the inserted one as they are merged.

Instead, if the inserted interval overlaps, just start merging.  The only time
we insert into the middle of the vector is when we don't overlap at all.  Also
delete blocks of live ranges if we overlap with many of them.

This patch speeds up joining by .7 seconds on a large testcase, but more
importantly gets all of the range adding code into addRangeFrom.

llvm-svn: 15141

comparisons, reducing linscan by another .1 seconds :)

llvm-svn: 15139

a very modest speedup of .3 seconds compiling 176.gcc (out of 20s).

llvm-svn: 15136

will soon be renamed) into their own file.  The new file should not emit
DEBUG output or have other side effects.  The LiveInterval class also now
doesn't know whether its working on registers or some other thing.

In the future we will want to use the LiveInterval class and friends to do
stack packing.  In addition to a code simplification, this will allow us to
do it more easily.

llvm-svn: 15134