[llvm-exegesis] Skip codegen of known-invalid snippets

On some targets, not all types of instruction operands are currently
handled. Instead of stopping the whole llvm-exegesis run because of any
instruction opcode that is not fully supported, write a per-opcode error
message and proceed to other opcodes. This improves the reliability of
--opcode-index=-1 sweep on partially supported targets.

Depends on: D146302, D146303

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Huawei RRI, OS Lab

Reviewed By: courbet

Differential Revision: https://reviews.llvm.org/D146304