In parseMSRMaskOperand, add an explicit check for the operand being an identifier instead of just having an assert.
In parseMSRMaskOperand, add an explicit check for the operand being an identifier instead of just having an assert.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@165480 91177308-0d34-0410-b5e6-96231b3b80d8

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