arch: Properly guess OpClass from optional StaticInst flags
arch: Properly guess OpClass from optional StaticInst flags

isa_parser.py guesses the OpClass if none were given based upon the StaticInst
flags.  The existing code does not take into account optionally set flags.
This code hoists the setting of optional flags so OpClass is properly assigned.
(transplanted from 43516d8eabe9b4e094d8c3755e8406eb08063a08)

LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5