Optimize reserved register coalescing.

Reserved registers don't have proper live ranges, their LiveInterval
simply has a snippet of liveness for each def.  Virtual registers with a
single value that is a copy of a reserved register (typically %esp) can
be coalesced with the reserved register if the live range doesn't
overlap any reserved register defs.

When coalescing with a reserved register, don't modify the reserved
register live range.  Just leave it as a bunch of dead defs.  This
eliminates quadratic coalescer behavior in i386 functions with many
function calls.

PR11699

llvm-svn: 147726