[mlir][emitc] Simplify inlining logic (NFCI) (#169978)
This change makes inlining logic in the translator simpler and more
consistent by
(a) Extending the inlining concept to include CExpression ops, which by
definition are inlined if and only if they reside within an
ExpressionOp.
(b) Concentraing all inlining decisions in `shouldBeInlined()` to make
sure that ops get the same decision when queried as operations and
as operands.
