IR: Clean up some duplicated code in ConstantDataSequential creation. NFC
IR: Clean up some duplicated code in ConstantDataSequential creation. NFC

ConstantDataArray::getImpl and ConstantDataVector::getImpl had a lot
of copy pasta in how they handled sequences of constants. Break that
out into a couple of simple functions.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@254456 91177308-0d34-0410-b5e6-96231b3b80d8

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