#435 readbyte() speedup for common cases

[Jindrich Makovicka]

The readbyte() function constantly checks if any of the mapped interfaces is enabled. It is more efficient to check the address, which is present in a register anyway, and only check the interfaces if the address is under 0x4000.

On R-Type RZX replay, the number of retired instructions drops from 20.1M to 16.5M (from 13.8% to 11.1% of total) as seen in the callgrind dump below.

--------------------------------------------------------------------------------
Ir                       file:function
--------------------------------------------------------------------------------
before:
45,498,785,091 (31.03%)  z80/z80_ops.c:z80_do_opcodes [/home/henry/build/fuse-emulator-fuse/fuse]
20,271,656,484 (13.82%)  memory_pages.c:readbyte [/home/henry/build/fuse-emulator-fuse/fuse]
12,135,737,120 ( 8.28%)  ./z80/opcodes_base.c:z80_do_opcodes

after:
45,375,286,300 (30.57%)  z80/z80_ops.c:z80_do_opcodes [/home/henry/build/fuse-emulator-fuse/fuse]
16,531,600,685 (11.14%)  memory_pages.c:readbyte [/home/henry/build/fuse-emulator-fuse/fuse]
12,085,983,272 ( 8.14%)  ./z80/opcodes_base.c:z80_do_opcodes

[Jindrich Makovicka]

the same patch with (hopefully) fixed coding style.

[Fredrick Meunier]

Thanks! Committed in [fe8398].