Bug in jcxz/jecxz detection code

Created by: nextsilicon-itay-bookstein

Describe the bug Running x64 code containing jecxz (and, to my understanding, x86 code containing jcxz) leads to incorrect behavior/crashes due to subtly broken pattern-detection code in core/arch/x86/instr.c. In particular, when running an application that uses libomp under debug drrun, the following assert is hit because the generated jecxz+jmp+jmp are tagged as meta instructions, and therefore are not detected as the second exit-cti: https://github.com/DynamoRIO/dynamorio/blob/d275a9aee4e2260cda0b7ce3f5e47f6f1b668320/core/arch/interp.c#L6187-L6189

The particular piece of code (instruction, even) that triggered this when running under DynamoRIO is this: https://github.com/llvm/llvm-project/blob/3a76b8a538c0d20efe823f03a133c778e842270e/openmp/runtime/src/z_Linux_asm.S#L1112

To Reproduce I'm not sure about the precise minimal conditions under which the problem reproduces, because when I attempted to yank out the above assembly routine to a test app and run it, I did not encounter the bug. The repro I debugged involved running an application that uses OpenMP against a locally installed llvm-7 libomp.so. I suspect that I need to somehow trigger cbr inversion for that particular branch for the assert/crash to happen. But I've already got a fix and will open a PR attached to this issue.

The problem (ASSERT_NOT_REACHED) reproduces under vanilla drrun without any clients. I used -debug -loglevel 4 to diagnose the issue.

Expected behavior No ASSERTs, application runs to completion successfully.

Versions ASSERTs in freshly compiled (14/05/2020) master.