[BUG] arith.fptoui issue with negative loop bound

[zzzDavid]

Describe the bug
Negative loop bound fails at IR building stage.

To Reproduce

import allo
from allo.ir.types import int32

N = 256

def compute(
    x: int32[N],
    y: int32[N]
):
    for i in range(N-1, -1, -1):
        y[i+1] = x[i+1]


s = allo.customize(compute)
print(s.module)

Buggy output

Traceback (most recent call last):
  File "/work/shared/users/phd/nz264/machsuite-allo/viterbi/viterbi.py", line 63, in <module>
    mod = s.build()
          ^^^^^^^^^
  File "/work/shared/users/phd/nz264/allo/allo/customize.py", line 682, in build
    return LLVMModule(
           ^^^^^^^^^^^
  File "/work/shared/users/phd/nz264/allo/allo/backend/llvm.py", line 59, in __init__
    self.module = Module.parse(str(mod), ctx)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^
hcl_mlir._mlir_libs._site_initialize.<locals>.MLIRError: Unable to parse module assembly:
error: "-":154:11: 'arith.fptoui' op result #0 must be signless-fixed-width-integer-like, but got 'index'
 note: "-":154:11: see current operation: %30 = "arith.fptoui"(%25) : (f32) -> index