nnc: fix Store if value is fp32 while buf is bf16 (#86788)
Fixes https://github.com/pytorch/pytorch/issues/86533.
For the below graph:
```bash
[DUMP kernel.cpp:1690] TensorExprKernel graph:
[DUMP kernel.cpp:1690] graph(%x.1 : BFloat16(10, strides=[1], requires_grad=0, device=cpu)):
[DUMP kernel.cpp:1690] %1 : int = prim::Constant[value=0]()
[DUMP kernel.cpp:1690] %2 : BFloat16(10, strides=[1], requires_grad=0, device=cpu) = aten::pow(%x.1, %1) # test/test_tensorexpr.py:1330:29
[DUMP kernel.cpp:1690] %3 : BFloat16(10, strides=[1], requires_grad=0, device=cpu) = aten::sin(%2) # test/test_tensorexpr.py:1330:19
[DUMP kernel.cpp:1690] return (%3)
```
**Loop stmt before the fix:**
The store value `0.8414709568023682f` is float while the scalar_type of the store buf `aten_sin` is bf16.
```bash
[DEBUG llvm_codegen.cpp:489] After HalfRewriter {
[DEBUG llvm_codegen.cpp:489] aten_sin[Ramp(0ll, 1ll, 8)] = Broadcast(0.8414709568023682f, 8);
[DEBUG llvm_codegen.cpp:489] for (int64_t i_1_tail_tail = 0ll; i_1_tail_tail < 2ll; i_1_tail_tail++) {
[DEBUG llvm_codegen.cpp:489] aten_sin[i_1_tail_tail + 8ll] = 0.8414709568023682f;
[DEBUG llvm_codegen.cpp:489] }
[DEBUG llvm_codegen.cpp:489] }
```
**Loop stmt after the fix:**
```bash
[DEBUG llvm_codegen.cpp:489] After HalfRewriter {
[DEBUG llvm_codegen.cpp:489] aten_sin[Ramp(0ll, 1ll, 8)] = bfloat16(Broadcast(0.8414709568023682f, 8));
[DEBUG llvm_codegen.cpp:489] for (int64_t i_1_tail_tail = 0ll; i_1_tail_tail < 2ll; i_1_tail_tail++) {
[DEBUG llvm_codegen.cpp:489] aten_sin[i_1_tail_tail + 8ll] = bfloat16(0.8414709568023682f);
[DEBUG llvm_codegen.cpp:489] }
[DEBUG llvm_codegen.cpp:489] }
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/86788
Approved by: https://github.com/EikanWang, https://github.com/kit1980
