[spirv] Add spv.loop

SPIR-V can explicitly declare structured control-flow constructs using merge
instructions. These explicitly declare a header block before the control
flow diverges and a merge block where control flow subsequently converges.
These blocks delimit constructs that must nest, and can only be entered
and exited in structured ways.

Instead of having a `spv.LoopMerge` op to directly model loop merge
instruction for indicating the merge and continue target, we use regions
to delimit the boundary of the loop: the merge target is the next op
following the `spv.loop` op and the continue target is the block that
has a back-edge pointing to the entry block inside the `spv.loop`'s region.
This way it's easier to discover all blocks belonging to a construct and
it plays nicer with the MLIR system.

Updated the SPIR-V.md doc.

PiperOrigin-RevId: 267431010