JS: copyMutableByteArray# is probably wrong

I noticed while working on !9629 (closed) that StgToJS appears to generate code for CopyMutableByteArrayOp that copies the bytes one-by-one from back to front. This is wrong if a user makes a call like copyMutableByteArray# marr# 32# marr# 0# 96#, because some locations will be written to as destination bytes before they are read from as source bytes.

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