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/x64/inst/mod.rs

@@ -25,7 +25,7 @@ mod emit_tests;
 pub mod regs;
 
 use args::*;
-use regs::{create_reg_universe_systemv, show_ireg_sized};
+use regs::show_ireg_sized;
 
 //=============================================================================
 // Instructions (top level): definition
@@ -943,10 +943,6 @@ impl MachInst for Inst {
             _ => {}
         }
     }
-
-    fn reg_universe(flags: &settings::Flags) -> RealRegUniverse {
-        create_reg_universe_systemv(flags)
-    }
 }
 
 impl<O: MachSectionOutput> MachInstEmit<O> for Inst {