ac/llvm: add AC_FLOAT_MODE_ROUND_TO_ZERO

Because some instructions will be optimized by the backend compiler,
the driver has to manually flush to zero to keep the result exact.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>

src/amd/llvm/ac_llvm_build.h

@@ -120,6 +120,8 @@ struct ac_llvm_context {
 	unsigned wave_size;
 	unsigned ballot_mask_bits;
 
+	unsigned float_mode;
+
 	LLVMValueRef lds;
 };