[mips][msa] lowerMSABitClear() should use SelectionDAG::getNOT() instead of...

[mips][msa] lowerMSABitClear() should use SelectionDAG::getNOT() instead of using a long-winded equivalent.

Now that getConstant(-1, MVT::v2i64) works correctly on MIPS32 we can use
SelectionDAG::getNOT() to produce the bitmask.

llvm-svn: 194819