Add spv.specConstant and spv._reference_of

Similar to global variables, specialization constants also live
in the module scope and can be referenced by instructions in
functions in native SPIR-V. A direct modelling would be to allow
functions in the SPIR-V dialect to implicit capture, but it means
we are losing the ability to write passes for Functions. While
in SPIR-V normally we want to process the module as a whole,
it's not common to see multiple functions get used so we'd like
to leave the door open for those cases. Therefore, similar to
global variables, we introduce spv.specConstant to model three
SPIR-V instructions: OpSpecConstantTrue, OpSpecConstantFalse,
and OpSpecConstant. They do not return SSA value results;
instead they have symbols and can only be referenced by the
symbols. To use it in a function, we need to have another op
spv._reference_of to turn the symbol into an SSA value. This
breaks the tie and makes functions still explicit capture.
Previously specialization constants were handled similarly as
normal constants. That is incorrect given that specialization
constant actually acts more like variable (without need to
load and store). E.g., they cannot be de-duplicated like normal
constants.

This CL also refines various documents and comments.

PiperOrigin-RevId: 264455172

0 files changed, 0 insertions, 0 deletions