EDSC: clean up type casting mechanism

Originally, edsc::Expr had a long enum edsc::ExprKind with all supported types
of operations.  Recent Expr extensibility support removed the need to specify
supported types in advance.  Replace the no-longer-used blocks of enum values
reserved for unary/binary/ternary/variadic expressions with simple values (it
is still useful to know if an expression is, e.g., binary to access it through
a simpler API).

Furthermore, wrap string-comparison now used to identify specific ops into an
`Expr::is_op<>` function template, that acts similarly to `Instruction::isa<>`.
Introduce `{Unary,Binary,Ternary,Variadic}Expr::make<> ` function template that
creates a Expression emitting the MLIR Op specified as template argument.

PiperOrigin-RevId: 234612916