nir: Add optimizations for iadd3

This patch also adds has_iadd3 bit to give more control if backend
supports ternary add instruction or not.

v2:
- Add patterns in late optimization (Connor Abbott)

Suggested-by: Alyssa/Jason

Signed-off-by: Sagar Ghuge <sagar.ghuge@intel.com>
Reviewed-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11596>

src/compiler/nir/nir.h

@@ -3433,6 +3433,9 @@ typedef struct nir_shader_compiler_options {
    /* Lowers when rotate instruction is not supported */
    bool lower_rotate;
 
+   /** Backend supports ternary addition */
+   bool has_iadd3;
+
    /**
     * Backend supports imul24, and would like to use it (when possible)
     * for address/offset calculation.  If true, driver should call

src/compiler/nir/nir_opt_algebraic.py

@@ -2269,6 +2269,10 @@ late_optimizations = [
    (('~fadd@32', ('fmul', a, b), c), ('ffma', a, b, c), 'options->fuse_ffma32'),
    (('~fadd@64', ('fmul', a, b), c), ('ffma', a, b, c), 'options->fuse_ffma64'),
 
+   (('iadd', ('iadd(is_used_once)', 'a(is_not_const)', 'b(is_not_const)'), 'c(is_not_const)'), ('iadd3', a, b, c), 'options->has_iadd3'),
+   (('iadd', ('isub(is_used_once)', 'a(is_not_const)', 'b(is_not_const)'), 'c(is_not_const)'), ('iadd3', a, ('ineg', b), c), 'options->has_iadd3'),
+   (('isub', ('isub(is_used_once)', 'a(is_not_const)', 'b(is_not_const)'), 'c(is_not_const)'), ('iadd3', a, ('ineg', b), ('ineg', c)), 'options->has_iadd3'),
+
    # These are duplicated from the main optimizations table.  The late
    # patterns that rearrange expressions like x - .5 < 0 to x < .5 can create
    # new patterns like these.  The patterns that compare with zero are removed