Replace 8-bit addition instructions for these individual registers with
16-bit instructions that affect the registers as pairs.

Change all assembly code using 16-bit addition to use the new
instructions. In some cases, this involves changing routines from using
other registers to using x34 and x78.

Before

CPU: user 33.63s system 0.31s, MEM: 337.59 MB total, 322.11 MB resident
Yosys 0.8 (git sha1 b003446, clang 7.0.1 -march=x86-64 -mtune=generic -O2 -fno-plt -fPIC -Os)
Time spent: 37% 26x opt_merge (12 sec), 36% 31x opt_expr (12 sec), ...

After placement:
PIOs       13 / 39
PLBs       446 / 660
BRAMs      8 / 30
  place time 19.02s

After

CPU: user 32.10s system 0.25s, MEM: 361.90 MB total, 337.75 MB resident
Yosys 0.8 (git sha1 b003446, clang 7.0.1 -march=x86-64 -mtune=generic -O2 -fno-plt -fPIC -Os)
Time spent: 36% 26x opt_merge (11 sec), 35% 31x opt_expr (11 sec), ...

After placement:
PIOs       13 / 39
PLBs       452 / 660
BRAMs      8 / 30
  place time 19.47s