[Builder] Add index from/to Float/Fixed/UFixed type casting

[zzzDavid]

Description

This PR introduces expanding support for casting between Index, Fixed, and UFixed types in the Allo IR. This enhancement allows for seamless conversions between these types, which broadens the flexibility of type handling in the IR layer.

Problems

Previously, the Allo IR lacked direct support for conversions between Index and Fixed/UFixed types, limiting the flexibility in scenarios where such conversions are needed for operations. Attempting these conversions required workarounds or was unsupported, creating friction in code requiring dynamic type handling.

Proposed Solutions

• Added Index <-> Fixed/UFixed single-step conversions in the cast_map within ASTTransformer.
• Implemented new casting operations IntToFixedOp and FixedToIntOp as intermediary conversions to facilitate direct type transformations.
• Updated the IndexCastOp to handle Index -> Fixed/UFixed conversions.

Examples

Here’s a snippet of the updated behavior:

1. Index to Fixed/UFixed Casting:

   def kernel(a: index) -> Fixed:
       a_fixed: Fixed = a
       return a_fixed

2. Fixed/UFixed to Index Casting:

   def kernel(a: Fixed) -> index:
       a_idx: index = a
       return a_idx

Both conversions now produce correct IR output without errors, validating the new casting support.

Checklist

• PR's title starts with a category (e.g., [IR], [Builder])
• Changes are complete (i.e., finished coding on this PR)
• All changes have test coverage, with cases for both Index -> Fixed/UFixed and Fixed/UFixed -> Index conversions
• Code is well-documented

[chhzh123]

Mostly LGTM

[chhzh123]

where a is used here?

[chhzh123]

Can you also add float<->index testing?

[zzzDavid]

Thanks, I fixed the init value issue in the test case, and added two test cases for float <-> index casting

[chhzh123]

Thanks!