[docs][Bugpoint]Add notes about multiple crashes

Summary:
    When reducing case for a CodeGenCrash, bugpoint may generate a new
    reduced
    testcase that exposes/causes another crash or break something due to
    limitation.

    Bugpoint does not distiguish different crashes currently,
    so when this happens, bugpoint will go on reducing for the new crash,
    or just abort, we can't get the case reduced for the origial crash.

    An advice is added into usage doc to connect to recommend checking error
    message with scripts and `-compile-command`.

Reviewers: modocache, bogner, sebpop, reames, vsk, MatzeB

Reviewed By: vsk

Subscribers: mehdi_amini, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66832

llvm-svn: 372157