Change live variables from using multimaps to using maps of vectors and

rearrange some of the accessors to be more efficient.

This makes it much more efficient to iterate over all of the things with the
same value.  This speeds up liveintervals analysis from 8.63s to 3.79s with
a release build of llc on kc++ with -march=ia64.  This also speeds up live
var from 1.66s -> 0.87s as well, reducing total llc time from 20.1s->15.2s.

This also speeds up other targets slightly, e.g. llc time on X86 from 16.84
-> 16.45s, and PPC from 17.64->17.03s.

llvm-svn: 22990