fix: make sure that multiple CHECK-NOT lines are all checked

[regehr]

currently only the first CHECK-NOT directive in our test files is actually checked.

for example, run this lit test in our current repo and it passes! of course it should not pass. it does not pass after this PR is merged.

; TEST-ARGS: -quiet

define i32 @src(i32 %x) {
  ret i32 %x
}

define i32 @tgt(i32 %x) {
  %y = add i32 %x, 0
  ret i32 %y
}

; CHECK-NOT: bla-this-string-never appears
; CHECK-NOT: Transformation seems to be correct!