[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).

Author: asb

llvm/lib/Target/RISCV/RISCVInstrInfo.cpp

@@ -2282,9 +2282,14 @@ 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, but limit the maximum ClusterSize to avoid creating too much
+  // additional register pressure.
+  return ClusterSize <= 4 && std::abs(Offset1 - Offset2) < CacheLineSize;
 }
 
 // Set BaseReg (the base register operand), Offset (the byte offset being