expr: Fix up the divmod cost debugging note [PR115910]

Something I've noticed during working on the crc wrong-code fix.
My first version of the patch failed because of no longer matching some
expected strings in the assembly, so I had to add TDF_DETAILS debugging
into the -fdump-rtl-expand-details dump which the crc tests can use.

For PR115910 Andrew has added similar note for the division/modulo case
if it is positive and we can choose either unsigned or signed
division.  The problem is that unlike most other TDF_DETAILS diagnostics,
this is not done before emitting the IL for the function, but during it.

Other messages there are prefixed with ;;, both details on what it is doing
and the GIMPLE IL for which it expands RTL, so the
;; Generating RTL for gimple basic block 4

;;

(code_label 13 12 14 2 (nil) [0 uses])

(note 14 13 0 NOTE_INSN_BASIC_BLOCK)
positive division: unsigned cost: 30; signed cost: 28

;; return _4;

message in between just looks weird and IMHO should be ;; prefixed.

2025-01-13  Jakub Jelinek  <jakub@redhat.com>

	PR target/115910
	* expr.cc (expand_expr_divmod): Prefix the TDF_DETAILS note with
	";; " and add a space before (needed tie breaker).  Formatting fixes.