Mode-Switching: Fix SET_SRC ICE for create_pre_exit

In same cases, like gcc/testsuite/gcc.dg/pr78148.c in RISC-V, there will
be only 1 operand when SET_SRC in create_pre_exit. For example as below.

(insn 13 9 14 2 (clobber (reg/i:TI 10 a0)) "gcc/testsuite/gcc.dg/pr78148.c":24:1 -1
  (expr_list:REG_UNUSED (reg/i:TI 10 a0)
    (nil)))

Unfortunately, SET_SRC requires at least 2 operands and then Segment
Fault here. For SH4 part result in Segment Fault, it looks like only
valid when the return_copy_pat is load or something like that. Thus,
this patch try to fix it by restrict the SET insn for SET_SRC.

Signed-off-by: Pan Li <pan2.li@intel.com>

gcc/ChangeLog:

	* mode-switching.cc (create_pre_exit): Add SET insn check.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/mode-switch-ice-1.c: New test.