Cache intermediate results during traceSiblingValue.

In some cases such as interpreters using indirectbr, the CFG can be very
complicated, and live range splitting may be forced to insert a large
number of phi-defs.  When that happens, traceSiblingValue can spend a
lot of time zipping around in the CFG looking for defs and reloads.

This patch causes more information to be cached in SibValues, and the
cached values are used to terminate searches early.  This speeds up
spilling by 20x in one interpreter test case.  For more typical code,
this is just a 10% speedup of spilling.

llvm-svn: 139247