nir: Add lowering for find_lsb.

There is a fairly simple relation to turn this into ufind_msb.

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

src/compiler/nir/nir.h

@@ -1915,6 +1915,8 @@ typedef struct nir_shader_compiler_options {
    bool lower_bfm;
    /** Lowers ifind_msb to compare and ufind_msb */
    bool lower_ifind_msb;
+   /** Lowers find_lsb to ufind_msb and logic ops */
+   bool lower_find_lsb;
    bool lower_uadd_carry;
    bool lower_usub_borrow;
    /** lowers fneg and ineg to fsub and isub. */