[PowerPC] set optimization level in SelectionDAGISel

[PowerPC] set optimization level in SelectionDAGISel

PowerPC backend does not pass the current optimization level to SelectionDAGISel and so SelectionDAGISel works with the default optimization level regardless of the current optimization level.
This patch makes the PowerPC backend set the optimization level correctly.

Differential Revision: https://reviews.llvm.org/D34615

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@306367 91177308-0d34-0410-b5e6-96231b3b80d8