[opt] [refactor] [bug] Avoid throwing exceptions in whole_kernel_cse and fix a bug

[xumingkuan]

Related issue = #1059

Fix a bug:

$1 = ...
$2 : if $1 {
  $3 = const [0]
} else {
  $4 = const [0]
}

This pass would move the first statements of both branches outside the clause and crash when checking if the last statements of both branches are the same.

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[yuanming-hu]

Thanks! LGTM!