nir/inline_functions: Break inlining into a builder helper

This pulls the guts of function inlining into a builder helper so that
it can be used elsewhere.  The rest of the infrastructure is still
needed for most inlining cases to ensure that everything gets inlined
and only ever once.  However, there are use-cases where you just want to
inline one little thing.  This new helper also has a neat trick where it
can seamlessly inline a function from one nir_shader into another.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/compiler/nir/nir.h

@@ -2807,7 +2807,8 @@ void nir_print_instr(const nir_instr *instr, FILE *fp);
 void nir_print_deref(const nir_deref_instr *deref, FILE *fp);
 
 nir_shader *nir_shader_clone(void *mem_ctx, const nir_shader *s);
-nir_function_impl *nir_function_impl_clone(const nir_function_impl *fi);
+nir_function_impl *nir_function_impl_clone(nir_shader *shader,
+                                           const nir_function_impl *fi);
 nir_constant *nir_constant_clone(const nir_constant *c, nir_variable *var);
 nir_variable *nir_variable_clone(const nir_variable *c, nir_shader *shader);
 
@@ -2940,6 +2941,9 @@ bool nir_split_struct_vars(nir_shader *shader, nir_variable_mode modes);
 bool nir_lower_returns_impl(nir_function_impl *impl);
 bool nir_lower_returns(nir_shader *shader);
 
+void nir_inline_function_impl(struct nir_builder *b,
+                              const nir_function_impl *impl,
+                              nir_ssa_def **params);
 bool nir_inline_functions(nir_shader *shader);
 
 bool nir_propagate_invariant(nir_shader *shader);