[LowerToHW] Avoid "assert/assume/cover" string comparison, refactor label creation, NFC

[uenoku]

Stop using "assert/assume/cover" string to specify a kind. Also factored out label creation.

Alternatively I considered to use declarative trait-based abstraction (like below) but did't like it because of heavy usage of templates, though it's still unfortunate to see there are 4 different target ops.

struct AssertVerif {
  using ImmediateOp = sv::AssertOp;
  using ConcurrentOp = sv::AssertConcurrentOp;
  using ClockedOp = verif::ClockedAssertOp;
  using VerifImmediateOp = verif::AssertOp;

  static constexpr StringLiteral labelPrefix = "assert__";
  static constexpr bool isCover = false;
  static constexpr bool supportsIfElseFatal = true;
};

Assisted-by: Codex: GPT 5.4