Fix the arm assembler so that this malformed instruction:
    ldrd r6, r7 [r2, #15]
simply gives an error and does not triggers an assertion.

As Jim points out, the diagnostic is really strange here,
but fixing that would be more complicated. The missing
comma results in the parser expecting a construct like r2[2],
which is the vector index thing the error message is talking
about. That's not what the user intended, though, and there's
nothing else in the instruction that looks at all like a vector.
Yet more fallout from not having a real parser here and trying
to do context-free generic matching for addressing modes.

rdar://15097243