nir/search: Fix is_16_bits for vectors

Require that all elements of a vector be representable as either
int16_t or uint16_t.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Fixes: 7ef45e661f ("intel/fs: Add constant propagation for ADD3")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29148>

src/compiler/nir/nir_search_helpers.h

@@ -355,12 +355,30 @@ is_16_bits(UNUSED struct hash_table *ht, const nir_alu_instr *instr,
    if (!nir_src_is_const(instr->src[src].src))
       return false;
 
+   /* All elements must be representable as int16_t or uint16_t. */
+   bool must_be_signed = false;
+   bool must_be_unsigned = false;
+
    for (unsigned i = 0; i < num_components; i++) {
       const int64_t val =
          nir_src_comp_as_int(instr->src[src].src, swizzle[i]);
 
       if (val > 0xffff || val < -0x8000)
          return false;
+
+      if (val < 0) {
+         if (must_be_unsigned)
+            return false;
+
+         must_be_signed = true;
+      }
+
+      if (val > 0x7fff) {
+         if (must_be_signed)
+            return false;
+
+         must_be_unsigned = true;
+      }
    }
 
    return true;