[RISCV] Change heuristic used for load clustering

Split out from #73789. 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.

Author: asb

llvm/lib/Target/RISCV/RISCVInstrInfo.cpp

@@ -2282,9 +2282,13 @@ bool RISCVInstrInfo::shouldClusterMemOps(
     return false;
   }
 
-  // TODO: Use a more carefully chosen heuristic, e.g. only cluster if offsets
-  // indicate they likely share a cache line.
-  return ClusterSize <= 4;
+  unsigned CacheLineSize =
+      BaseOps1.front()->getParent()->getMF()->getSubtarget().getCacheLineSize();
+  // Assume a cache line size of 64 bytes if no size is set in RISCVSubtarget.
+  CacheLineSize = CacheLineSize ? CacheLineSize : 64;
+  // Cluster if the memory operations are on the same or a neighbouring cache
+  // line.
+  return std::abs(Offset1 - Offset2) < CacheLineSize;
 }
 
 // Set BaseReg (the base register operand), Offset (the byte offset being