Previously, any blank lines in IR were ignored by UTC, leading to more
fragile `CHECK`s being generated.
This change lets UTC, 1) emit `CHECK-EMPTY` to check blank lines, and 2)
generate more `CHECK-NEXT`s, landing the discussion
https://github.com/llvm/llvm-project/pull/165419#issuecomment-3457572422.
Moreover, this change also aligns the behavior of IR check-gen to ASM
check-gen, which has been emitting `CHECK-EMPTY` since
a8a89c77ea.
82cf54fbf6