nir: Add lowering for ifind_msb to ufind_msb.

ufind_msb is easily expressed in terms of clz, and we can reduce ifind_msb
to that.

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 bfm to shifts and subtracts. */
    bool lower_bfm;
+   /** Lowers ifind_msb to compare and ufind_msb */
+   bool lower_ifind_msb;
    bool lower_uadd_carry;
    bool lower_usub_borrow;
    /** lowers fneg and ineg to fsub and isub. */