Improve shrink wrapping debug output

Improve shrink wrapping debug output

Currently if requires_stack_frame_p() returns true for some insn, the
shrink-wrapping debug output contains only the number of a block containing
that insn.

But it is very useful to see the particular insn that requires the prologue.
Let's call print_rtl_single to display that insn in the following pass dump.

gcc/

	* shrink-wrap.c (try_shrink_wrapping): Improve debug output.