slim up mem exprs to avoid line breaks in -fverbose-asm

slim up mem exprs to avoid line breaks in -fverbose-asm

An asm operand with a "VIEW_CONVERT_EXPR<struct {
  [...]
}>" will output the definition of the struct as asm code.  Oops.

Enable TDF_SLIM in print_mem_expr to avoid such line breaks.

for  gcc/ChangeLog

	* print-rtl.c (print_mem_expr): Enable TDF_SLIM in dump_flags.