core: Do addpcis using the main adder (#189)

core: Do addpcis using the main adder (#189)

By adding logic to decode2 to be able to send the instruction address
down the A input, and making CONST_DX_HI (renamed to CONST_DXHI4) add
4 to the immediate value (easy since the bottom 16 bits were zero),
we can do addpcis using the main adder.  This reduces the width of the
result mux and frees up one value in insn_type_t, since we can now use
OP_ADD for addpcis.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>