nir: Add lowering for nir_op_bit_count.

This is basically the same as the GLSL lowering path.

v2: Fix typo in the link

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/compiler/nir/nir.h

@@ -1913,6 +1913,8 @@ typedef struct nir_shader_compiler_options {
    bool lower_bitfield_insert_to_shifts;
    /** Lowers bitfield_reverse to shifts. */
    bool lower_bitfield_reverse;
+   /** Lowers bit_count to shifts. */
+   bool lower_bit_count;
    /** Lowers bfm to shifts and subtracts. */
    bool lower_bfm;
    /** Lowers ifind_msb to compare and ufind_msb */