Fix nested _Rational in Pow codegen for negative exponents

When Pow(X, -1) is codegen'd and X contains a division (producing a
_Rational on the stack), the base must be resolved to an MLIR value
before wrapping in a new _Rational. Otherwise a nested _Rational
propagates to floor/floordiv_expr which expects flat numerator and
denominator values.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Ivan Butygin <ivan.butygin@gmail.com>

Author: Hardcode84

wave_lang/kernel/compiler/wave_codegen/emitter.py

@@ -1097,6 +1097,7 @@ def _get_const(val):
                     operand = _get_ir_value(base)
                     base = arith_d.muli(operand, operand)
                 if power < 0:
+                    base = _resolve_rational(base)
                     stack.append(_Rational(_get_const(1), base))
                 else:
                     stack.append(base)