Better implementation of %SIGNED-MULTIPLY-HIGH and signed truncate transform.

 - Add VOPs. The PPC and ARM VOPs haven't been tested, but only differ
   slightly from their %MULTIPLY-HIGH counterparts, so should in theory
   work fine.
 - 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.