glasm: Catch more register leaks

Add support for null registers. These are used when an instruction has
no usages.

This comes handy when an instruction is only used for its CC value, with
the caveat of having to invalidate all pseudo-instructions before
defining the instruction itself in the register allocator. This commits
changes this.

Workaround a bug on Nvidia's condition codes conditional execution using
branches.

src/shader_recompiler/backend/glasm/emit_glasm_composite.cpp

@@ -52,7 +52,9 @@ void CompositeInsert(EmitContext& ctx, IR::Inst& inst, Register composite, Objec
         // The input composite is not aliased with the return value so we have to copy it before
         // hand. But the insert object is not aliased with the return value, so we don't have to
         // worry about that
-        ctx.Add("MOV.{} {},{};MOV.{} {}.{},{};", type, ret, composite, type, ret, swizzle, object);
+        ctx.Add("MOV.{} {},{};"
+                "MOV.{} {}.{},{};",
+                type, ret, composite, type, ret, swizzle, object);
     } else {
         // The return value is alised so we can just insert the object, it doesn't matter if it's
         // aliased