common.nativeexec_exe_opt: Usage error: get_implied_mm_e_vex_opcode_bytes: internal prefix error

See https://travis-ci.com/DynamoRIO/dynamorio/jobs/219331286

Test common.nativeexec_exe_opt ran into the following assertion that must not happen:

<Usage error: get_implied_mm_e_vex_opcode_bytes: internal prefix error (/home/travis/build/DynamoRIO/dynamorio/core/arch/x86/decode_fast.c, line 1280)
version 7.91.18102, custom build

This means the mm bits in the fast decoder were invalid. Unclear why it happened.