FIXES ISSUE #473 : lowest/highest_set_bit functions should return a Bits type?

[omsuneri]

This PR fixes #473.
The lowest_set_bit and highest_set_bit functions were returning XReg (i.e., Bits) though they only compute a position index. Updated to return Bits<8> instead, which is sufficient and semantically correct.
All smoke tests and IDL validations pass successfully.
@ThinkOpenly please review this and request if any further chnaged are required !!

[dhower-qc]

One thing to check here:

If the return of these functions is fed into an operator that might lose information from insufficient bit width, this could introduce a subtle bug. For example:

Bits<8> a = 128;
XReg b = a << 1;   # b = 0

XReg c = 128;
XReg d = c << 1;   # d = 256
---

Can you check how the returns are currently being used?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 43.30%. Comparing base (77d4edc) to head (51065af).
Report is 3 commits behind head on main.

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[omsuneri]

@dhower-qc I tested the overflow behavior using tests/dev/test_shift_overflow.py

def simulate_bits8_shift():
    bits8 = 128
    result = (bits8 << 1) & 0xFF
    return result

def simulate_xreg_shift():
    xreg = 128
    result = xreg << 1 
    return result

def test_shift_behavior():
    bits8_result = simulate_bits8_shift()
    xreg_result = simulate_xreg_shift()

    print(f"Bits<8>: 128 << 1 = {bits8_result} (Expected: 0 if overflowed)")
    print(f"XReg   : 128 << 1 = {xreg_result} (Expected: 256)")

    assert bits8_result == 0, "Bits<8> incorrectly retained the value (should overflow)"
    assert xreg_result == 256, "XReg did not shift correctly"

if __name__ == "__main__":
    test_shift_behavior()

result:

root@08ff66764da5:/workspaces/riscv-unified-db# python3 tests/dev/test_shift_overflow.py
Bits<8>: 128 << 1 = 0 (Expected: 0 if overflowed)
XReg   : 128 << 1 = 256 (Expected: 256)

This confirms that using Bits<8> in return from lowest_set_bit / highest_set_bit may be unsafe unless all downstream uses are known to be non-shift, non-multiply....
what would you suggest me to do next !!