Merging r342946:

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r342946 | smaksimovic | 2018-09-24 23:27:49 -0700 (Mon, 24 Sep 2018) | 6 lines

[mips] Correct MUL pattern for mips64

Guard existing pattern with a predicate, introduce a new one for revision 6.

Differential Revision: https://reviews.llvm.org/D51684

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llvm-svn: 346742