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. */

src/compiler/nir/nir_opt_algebraic.py

@@ -555,6 +555,10 @@ optimizations = [
       ('bfm', 'bits', 0))),
     'options->lower_bitfield_extract_to_shifts'),
 
+   (('ifind_msb', 'value'),
+    ('ufind_msb', ('bcsel', ('ilt', 'value', 0), ('inot', 'value'), 'value')),
+    'options->lower_ifind_msb'),
+
    (('extract_i8', a, 'b@32'),
     ('ishr', ('ishl', a, ('imul', ('isub', 3, b), 8)), 24),
     'options->lower_extract_byte'),