[X86] Add WriteBitScan/WriteLZCNT/WriteTZCNT/WritePOPCNT scheduler classes (PR36881)

Give the bit count instructions their own scheduler classes instead of forcing them into existing classes.

These were mostly overridden anyway, but I had to add in costs from Agner for silvermont and znver1 and the Fam16h SoG for btver2 (Jaguar).

Differential Revision: https://reviews.llvm.org/D44879

llvm-svn: 328566