In opcode.h, use the alias macros in PL_ppaddr[]

In opcode.h, use the alias macros in PL_ppaddr[]

For OPs that share implementations, use #define aliases for the "original"
names in PL_ppaddr[], instead of having regen/opcode.pl replace them with
the name of the implementing OP body. Whilst this initially adds another layer
of indirection, it will give more flexibility where conditional compilation of
OPs is determined by the feature macros in config.h