[RegAlloc] Skip global splitting if the live range is huge and its spill is

trivially rematerializable.

We run into a case where machineLICM hoists a large number of live ranges
outside of a big loop because it thinks those live ranges are trivially
rematerializable. In regalloc, global splitting is tried out first for those
live ranges before they are spilled and rematerialized. Because the global
splitting algorithm is quadratic, increasing a lot of global splitting
candidates causes huge compile time increase (50s to 1400s on my local
machine when compiling a module).

However, we think for live ranges which are very large and are trivially
rematerialiable, it is better to just skip global splitting so as to save
compile time with little chance of sacrificing performance.  We uses the
segment size of live range to indirectly evaluate whether the global
splitting of the live range can introduce high cost, and use an option
as a knob to adjust the size limit threshold.

Differential Revision: https://reviews.llvm.org/D49353

llvm-svn: 337186