* combine.c (struct reg_stat): Add new fields truncation_label and
truncated_to_mode.
(record_value_for_reg): Reset truncated_to_mode.
(record_truncated_value): New function.
(check_promoted_subreg): Call it. Rename to check_conversions.
(combine_instructions): Rename check_promoted_subreg to
check_conversions.
(reg_truncated_to_mode): New function.
(make_extraction): Use it. Check TRULY_NOOP_TRUNCATION.
(gen_lowpart_or_truncate): New function.
(force_to_mode): Use it instead of gen_lowpart.
From-SVN: r109679
