[mips] Custom-lower i64 MULHS and MULHU nodes. Remove the code which selects
multiply instructions in MipsSEDAGToDAGISel.

This patch was supposed to be part of r178403.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179314 91177308-0d34-0410-b5e6-96231b3b80d8