Add mechanism to specify extended instruction sets in SPIR-V.

Add support for specifying extended instructions sets. The operations
in SPIR-V dialect are named as 'spv.<extension-name>.<op-name>'. Use
this mechanism to define a 'Exp' operation from GLSL(450)
instructions.
Later CLs will add support for (de)serialization of these operations,
and update the dialect generation scripts to auto-generate the
specification using the spec directly.

Additional changes:
Add a Type Constraint to OpBase.td to check for vector of specified
lengths. This is used to check that the vector type used in SPIR-V
dialect are of lengths 2, 3 or 4.
Update SPIRVBase.td to use this Type constraints for vectors.

PiperOrigin-RevId: 269234377