Add test cases in the muldiv test for the range of the dividend.

Add a case that produced incorrect results before precision was fixed.
Also add random ranges.
Move transformations related to division by constants to a new file.

This makes it easier to manage the order in which the transforms
are tried.
Better implementation of %SIGNED-MULTIPLY-HIGH and signed truncate transform.

 - Add VOPs.
 - Add some TRULY-THEs to GEN-SIGNED-DIV-BY-CONSTANT-EXPR
   to use arithmetic operations on signed words.
 - Change the muldiv test to check for "MUL" in the case of
   signed division too.
Add examples for GEN-SIGNED-DIV-BY-CONST-EXPR average and worst case.

Also edit examples for the unsigned generator to use the correct function
names.