intel/vec4: Don't optimize multiply by 1.0 away

The SPIR-V compiler's implementation of tanh generates a multiply by 1.0
to flush denorms to zero.

Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20232>

src/intel/compiler/brw_vec4.cpp

@@ -668,6 +668,12 @@ vec4_visitor::opt_algebraic()
 	 break;
 
       case BRW_OPCODE_MUL:
+	 if (inst->src[1].file != IMM)
+	    continue;
+
+	 if (brw_reg_type_is_floating_point(inst->src[1].type))
+	    break;
+
 	 if (inst->src[1].is_zero()) {
 	    inst->opcode = BRW_OPCODE_MOV;
 	    switch (inst->src[0].type) {