compiler: on PPC, ANDI, ORI and XORI instructions take an unsigned 16-bit immediate...

compiler: on PPC, ANDI, ORI and XORI instructions take an unsigned 16-bit immediate, unlike ADDI, SUBI and MULLI which take a signed 16-bit immediate. The code generator was not aware of this, and so for example '[ >fixnum -16 bitand ]' would generate incorrect code. Split up small-enough? hook into immediate-arithmetic? and immediate-bitwise? and update value numbering to be aware of this. Fixes classes.struct bitfields test failure