RFD: hookizing BITS_PER_UNIT in tree optimizers / frontends

[Joern Rennecke <amylaar at spamcop dot net>]

If we changed BITS_PER_UNIT into an ordinary piece-of-data 'hook', this
would not only cost a data load from the target vector, but would also
inhibit optimizations that replace division / modulo / multiply with shift
or mask operations.
So maybe we should look into having a few functional hooks that do  
common operations, i.e.
bits_in_units        x / BITS_PER_UNIT
bits_in_units_ceil   (x + BITS_PER_UNIT - 1) / BITS_PER_UNIT
bit_unit_remainder   x % BITS_PER_UNIT
units_in_bits        x * BITS_PER_UNIT

Although we currently have some HOST_WIDE_INT uses, I hope using
unsigned HOST_WIDE_INT as the argument / return type will generally work.

tree.h also defines BITS_PER_UNIT_LOG, which (or its hook equivalent)
should probably be used in all the places that use
exact_log_2 (BITS_PER_UNIT), and, if it could be relied upon to exist, we
could also use it as a substitute for the above hooks.  However, this seems
a bit iffy - we'd permanently forgo the possibility to have 6 / 7 / 36
bit etc. units.

Similar arrangements could be made for BITS_PER_WORD and UNITS_PER_WORD,
although these macros seem not quite so prevalent in the tree optimizers.