[libc] Make the opcode parameter a compile time constant

Currently the opcode is only valid if it is the same between all of the
ports. This is possible to violate if the opcode is places into a memory
location and then read in a non-uniform manner by the warp / wavefront.
Moving this to a compile time constant makes it impossible to break this
invariant.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D150115