speed of the new mode checker

[Zoltan Somogyi]

I tested the speed of the compiler on the alias branch versus the one on the
main branch (with both compilers themselves compiled with the default
optimization level).

Using -O1 --no-common-struct --no-follow-code to compile make_hlds.m
(the highest optimization level that works on the alias branch on this module), the results on cyclone were:

50.282u 1.041s 0:52.21 98.2%    6+334k 14+149io 0pf+0w
50.185u 1.030s 0:51.64 99.1%    6+333k 0+146io 0pf+0w
50.515u 1.051s 0:52.34 98.5%    6+333k 16+142io 0pf+0w

on the alias branch and

39.066u 1.201s 0:47.53 84.7%    6+300k 492+144io 334pf+0w
38.840u 1.075s 0:40.54 98.4%    6+301k 0+142io 0pf+0w
38.546u 1.103s 0:41.77 94.9%    6+300k 33+143io 0pf+0w

on the main branch, so the new modechecker slows the compiler down only by
about 29%. Given that alias tracking should enable last call optimization
modulo construction and compile time garbage collection, which together should
yield more speedup than that, this is good news.

Zoltan.