[RISCV] Change heuristic used for load clustering (#75341)

Split out from #73789, so as to leave that PR just for flipping load
clustering to on by default. Clusters if the operations are within a
cache line of each other (as AMDGPU does in shouldScheduleLoadsNear).
X86 does something similar, but does `((Offset2 - Offset1) / 8 > 64)`.
I'm not sure if that's intentionally set to 512 bytes or if the division
is in error.

Adopts the suggestion from @wangpc-pp to query the cache line size and
use it if available.

We also cap the maximum cluster size to cap the potential register
pressure impact (which may lead to additional spills).