BitcodeReader: Consistently use IsDistinct, NFC

Consistently use the IsDistinct variable and start relying on it in
GET_OR_DISTINCT.  This change has NFC, but prepares for using IsDistinct
to optimize the behaviour of the getMD() and getMDOrNull() helpers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@267268 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed