Avoid generating value split instructions.

The legalizer often splits values into parts with the vsplit and
isplit_lohi instructions. Avoid doing that for values that are already
defined by the corresponding concatenation instructions.

This reduces the number of instructions created during legalization, and
it simplifies later optimizations. A number of dead concatenation
instructions are left behind. They can be trivially cleaned up by a dead
code elimination pass.

lib/cretonne/src/legalizer/mod.rs

@@ -18,6 +18,7 @@ use ir::condcodes::IntCC;
 use isa::{TargetIsa, Legalize};
 
 mod boundary;
+mod split;
 
 /// Legalize `func` for `isa`.
 ///