Irrefutable if expressions are not diagnosed

[tshortli]

Description

The compiler does not emit diagnostics about irrefutable conditions in if expressions, even though it does for if statements.

Reproduction

func testStmt() -> Bool {
  if case _: Int? = nil { // warning: 'if' condition is always true
    return true
  } else {
    return false
  }
}

func testExpr() -> Bool {
  return if case _: Int? = nil { true } else { false } // no warning
}

Expected behavior

Both test cases should be diagnosed equivalently.

Environment

$ ../build/Ninja-RelWithDebInfoAssert/swift-macosx-arm64/bin/swiftc -version
Swift version 6.3-dev (LLVM 82c14639a1e4d89, Swift e896e0b9b7cf78f)
Target: arm64-apple-macosx26.0
Build config: +assertions

Additional information

No response