[mips] SelectionDAGISel subclasses now follow the optimization level.

Summary:
It was recently discovered that, for Mips's SelectionDAGISel subclasses,
all optimization levels caused SelectionDAGISel to behave like -O2.

This change adds the necessary plumbing to initialize the optimization level.

Reviewers: andrew.w.kaylor

Subscribers: andrew.w.kaylor, sdardis, dean, llvm-commits, vradosavljevic, petarj, qcolombet, probinson, dsanders

Differential Revision: https://reviews.llvm.org/D14900

llvm-svn: 275410