[TableGen] Fix a potential crash when operand doesn't appear in the instruction pattern (#87663)

We have a check of whether an operand is in the instruction pattern, and
emit an
error if it is not, but we simply continue execution, including directly
dereferencing a point-like object `InVal`, which will be just created
when
accessing the map. It contains a `nullptr` so dereferencing it causes
crash.
This is a very trivial fix.