[XCore] Provide information about known zero bits of resource instructions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202393 91177308-0d34-0410-b5e6-96231b3b80d8
[XCore] Add dag combines for instructions that ignore some input bits.

These instructions ignore the high bits of one of their input operands -
try and use this to simplify the code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202394 91177308-0d34-0410-b5e6-96231b3b80d8