[Bug middle-end/122871] [13/14/15/16/17 Regression] de-optimized synthesis of long long shift and add

[roger at nextmovesoftware dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122871

--- Comment #16 from Roger Sayle <roger at nextmovesoftware dot com> ---
The slightly unusual results from synth_mult on ARM with -mthumb are due to
incorrect values being returned by rtx_costs.  DImode addition and subtraction
should both be COSTS_N_INSNS(2), i.e. adds+adcs and subs+sbcs respectively, but
currently return COSTS_N_INSNS(1).