[NFC][RISCV] Enable TuneNoDefaultUnroll feature to control targets which use default unroll preference

[zixuan-wu]

Summary

In RISCVTargetTransformInfo, enumerating the processor family is not a good way to predict. Because it needs to enumerate many subtarget family and is hard to update if add new subtarget. Instead, create a feature to distinguish whether targets want to use default unroll preference or not.

Keep TuneSiFive7 because it's flag to indicate subtarget family, which may used in other place.

[craig.topper]

llvm/lib/Target/RISCV/RISCV.td

337

Can you put this near the TuneSiFive7 flag and name it TuneNoDefaultUnroll.

[zixuan-wu]

Address comments to change feature name.

[craig.topper]

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp

356

"explicit targets" no longer makes sense here since you removed the flag and inverted the comment.

356

Oops I mean. "explicit targets" no longer makes sense here since you added the flag and inverted the comment.

[mcberg2021]

LGTM.