codegen: split lower.rs into multiple files;

This splits off lower.rs into two files: lower.rs keeps all the utility
functions, while lower_inst.rs contains the (gigantic!) function
lowering a single Cranelift instruction into vcode.

This is done to satisfy a check done on the maximal file's size when
vendoring Rust source code into Mozilla central's repository.

cranelift/codegen/src/isa/aarch64/mod.rs

@@ -17,6 +17,7 @@ use target_lexicon::{Aarch64Architecture, Architecture, Triple};
 mod abi;
 mod inst;
 mod lower;
+mod lower_inst;
 
 use inst::create_reg_universe;