MachInst backend: don't reallocate RealRegUniverses for each function

compilation.

This saves ~0.14% instruction count, ~0.18% allocated bytes, and ~1.5%
allocated blocks on a `clif-util wasm` compilation of `bz2.wasm` for
aarch64.

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

@@ -1903,10 +1903,6 @@ impl MachInst for Inst {
             _ => {}
         }
     }
-
-    fn reg_universe(flags: &settings::Flags) -> RealRegUniverse {
-        create_reg_universe(flags)
-    }
 }
 
 //=============================================================================