Move the temp allocation back to hlsl_codegen.c.
Note that the DCL_TEMPS instructions wouldn't be necessary if we had the
capacity to store the temp_count for both the main program and the patch
constant program (or more generally speaking, a temp_count for all
phases).
The plan is to eventually also move the HS_CONTROL_POINT and
HS_FORK_PHASE markers to the vsir_program, making it able to contain
both functions.