[MetaSchedule] Fix a multilevel tiling error on dynamic relax workload

[wrongtest-intellif]

Below script would either segfault or report primitive error in MetaScheduleTuneIRMod with dynamic dim.

import tvm
from tvm import relax
from tvm.relax.frontend import nn
from tvm.relax.transform import *

class NNModule(nn.Module):
    def __init__(self):
        super().__init__()
        self.fc2 = nn.Linear(784, 128)

    def forward(self, x):
        x = self.fc2(x)
        return x

mod, params = NNModule().export_tvm({"forward": {"x": nn.spec.Tensor(("n", 784), "float32")}})
mod.show()

# with tvm.target.Target("cuda -max_threads_per_block=4096 -max_shared_memory_per_block=8192 -arch=sm_70"):
with tvm.target.Target("llvm -num-cores 32"):
    mod = LegalizeOps()(mod)
    mod = AnnotateTIROpPattern()(mod)
    mod = FoldConstant()(mod)
    mod = FuseOps()(mod)
    mod = FuseTIR()(mod)
    mod = MetaScheduleTuneIRMod({}, "myworkingdir", 3)(mod)
    application_pass = MetaScheduleApplyDatabase( "myworkingdir")
    mod = application_pass(mod)
    
print(mod)
m = relax.build(mod, target="llvm", pipeline="default_build")

The change fix two related issues:

1. GetLoopIntExtent() would return nullptr on non-const extent, thus the return value should check the nullity.

2. tir::SamplePerfectTile() return -1 for dynamic factor part. While the Split primitive require None rv to denotes the dynamic dim inference. Thus it should handle this semantic gap in schedule interfaces.