EDSC: introduce support for blocks

EDSC currently implement a block as a statement that is itself a list of
statements.  This suffers from two modeling problems: (1) these blocks are not
addressable, i.e. one cannot create an instruction where thus constructed block
is a successor; (2) they support block nesting, which is not supported by MLIR
blocks.  Furthermore, emitting such "compound statement" (misleadingly named
`Block` in Python bindings) does not actually produce a new Block in the IR.

Implement support for creating actual IR Blocks in EDSC.  In particular, define
a new StmtBlock EDSC class that is neither an Expr nor a Stmt but contains a
list of Stmts.  Additionally, StmtBlock may have (early-) typed arguments.
These arguments are Bindable expressions that can be used inside the block.
Provide two calls in the MLIREmitter, `emitBlock` that actually emits a new
block and `emitBlockBody` that only emits the instructions contained in the
block without creating a new block.  In the latter case, the instructions must
not use block arguments.

Update Python bindings to make it clear when instruction emission happens
without creating a new block.

PiperOrigin-RevId: 234556474