[BasicTTI] Allow generic handling of scalable vector fshr/fshl

This change removes an explicit scalable vector bailout for fshl and fshr. This bailout was added in 60e4698b, when sinking a unconditional bailout for all intrinsics into selected cases. Its not clear if the bailout was originally unneeded, or if our cost model infrastructure has simply matured in the meantime. Either way, the generic code appears to handle scalable vectors without issue.

Note that the RISC-V cost model changes here aren't particularly interesting. They do probably better match the current lowering, but the main point is to have coverage of the BasicTTI path and simply show lack of crashing.

AArch64 costing was changed to preserve legacy behavior.  There will most likely be an upcoming change to use the generic costs there too, but I didn't want to make that change not being particularly familiar with the target.

Differential Revision: https://reviews.llvm.org/D127680