[TableGen] Don't assert, produce an error, when an instruction has too few operands

When an instruction's operand list does not have a sufficient number of
operands to match with all of the variables that contribute to its
encoding, instead of asserting inside a call to getSubOperandNumber, produce an
informative error.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204542 91177308-0d34-0410-b5e6-96231b3b80d8