An `emitc.expression` can only yield a single result, but some
operations which have the `CExpression` trait can have multiple results,
which can result in a crash when applying the `fold-expressions` pass.
This change adds a check for the single-result condition and a simple
test.
Add an emitc.expression operation that models C expressions, and provide
transforms to form and fold expressions. The translator emits the body
of
emitc.expression ops as a single C expression.
This expression is emitted by default as the RHS of an EmitC SSA value,
but if
possible, expressions with a single use that is not another expression
are
instead inlined. Specific expression's inlining can be fine tuned by
lowering
passes and transforms.
