Fix build for STORE_FLAG_VALUE<0 targets [PR118418]

In g:06c4cf39 I mishandled signed
comparisons of comparison results on STORE_FLAG_VALUE < 0 targets
(despite specifically referencing STORE_FLAG_VALUE in the commit
message).  There, (lt TRUE FALSE) is true, although (ltu FALSE TRUE)
still holds.

Things get messy with vector modes, and since those weren't the focus
of the original commit, it seemed better to punt on them for now.
However, punting means that this optimisation no longer feels like
a natural tail-call operation.  The patch therefore converts
"return simplify..." to the usual call-and-conditional-return pattern.

gcc/
	PR target/118418
	* simplify-rtx.cc (simplify_context::simplify_relational_operation_1):
	Take STORE_FLAG_VALUE into account when handling signed comparisons
	of comparison results.