[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


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@342946 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed