[MIPS MSA] Avoid some DAG combines for vector shifts

[MIPS MSA] Avoid some DAG combines for vector shifts

DAG combiner combines two shifts into shift + and with bitmask.
Avoid such combines for vectors since leaving two vector shifts
as they are produces better end results.

Differential Revision: https://reviews.llvm.org/D58225

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@354461 91177308-0d34-0410-b5e6-96231b3b80d8