re PR target/12900 (g++ ICE in rtl_verify_flow_info_1)
PR target/12900
* reg-stack (move_for_stack_reg): New prototype. Return
whether a control flow insn was deleted.
(subst_stack_regs_pat): Likewise, using the information provided
by move_for_stack_reg.
(subst_stack_regs): Likewise, using the information provided
by subst_stack_regs_pat.
(convert_regs_1): Record whether a control flow insn was deleted,
using the information provided by subst_stack_regs. Purge dead
edges only if a control flow insn was deleted.