Generate an INST_PREDICATES table for each ISA.

Instead of generating a single `check_instp()` function, create an array
of individual function pointers for checking instruction predicates.

This makes explicit the jump table in the old check_instp() method and
it gives us a way of determining the number of instruction predicates
that exists.

lib/cretonne/src/isa/arm32/enc_tables.rs

@@ -1,6 +1,5 @@
 //! Encoding tables for ARM32 ISA.
 
-use ir::InstructionData;
 use ir::types;
 use isa::EncInfo;
 use isa::constraints::*;