fold: Don't fold invalid addition/subtraction rather than failing

This may happen in a branch QBE doesn't realize is unreachable,
for example (simplified from real code found in ncurses)

	data $str = { b "abcdef", b 0 }
	function l $f(w %x) {
	@start
		%.1 =w ceqw %x, 0
		jnz %.1, @logic_join, @logic_right
	@logic_right
		%p =l call $strchr(l $str, w %x)
		%.2 =w ceql %p, 0
	@logic_join
		%.3 =w phi @start %.1, @logic_right %.2
		jnz %.3, @fail, @return
	@fail
		ret 0
	@return
		%.4 =l sub %p, $str
		ret %.4
	}

0 files changed, 0 insertions, 0 deletions