[llvm-mca] report an error if the assembly sequence contains an unsupported instruction.

This is a short-term fix for PR38093.
For now, we llvm::report_fatal_error if the instruction builder finds an
unsupported instruction in the instruction stream.

We need to revisit this fix once we start addressing PR38101.
Essentially, we need a better framework for error handling.

llvm-svn: 336543