[MachineSink] Compile time improvement for large testcases which has many kill flags

We did a experiment and observed dramatic decrease on compilation time which spent on clearing kill flags.
Before:
Number of BasicBlocks:33357
Number of Instructions:162067
Number of Cleared Kill Flags:32869
Time of handling kill flags(ms):1.607509e+05

After:
Number of BasicBlocks:33357
Number of Instructions:162067
Number of Cleared Kill Flags:32869
Time of handling kill flags:3.987371e+03

Reviewed By: MatzeB

Differential Revision: https://reviews.llvm.org/D111688