[GISel][TableGen] Enhance default ops support (#75689)

- Instead of checking the default ops directly, this change queries DAG
default operands collected during patterns reading. It does not only
simplify the code but also handle few cases where integer values are
converted from convertible types, such as 'bits'.
- A test case is added GlobalISelEmitter.td as the regression test of
default 'bits' values.