Fix assert about maximum fragment split size (#92027)

We want to support arbitrary `DOTNET_JitSplitFunctionSize` values
for stressing the unwind fragment splitting logic, but there are
limitations to the smallest size that can be split. Thus, we can
exceed a specified minimum. Loosen an assert that the last
fragment be less than the specified split size, since it might
exceed that minimum, e.g., by being a large epilog. Leave the assert
specifying the hard requirement of maximum fragment size.

src/coreclr/jit/emit.cpp

@@ -3143,7 +3143,7 @@ void emitter::emitSplit(emitLocation* startLoc,
  } // end for loop
 
  splitIfNecessary();
- assert(curSize < maxSplitSize);
+ assert(curSize < UW_MAX_FRAGMENT_SIZE_BYTES);
 }
 
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