Merging r314798:

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r314798 | sdardis | 2017-10-03 06:45:49 -0700 (Tue, 03 Oct 2017) | 9 lines

[mips] Enable spilling and reloading of the dsp register set.

The dsp register class is an alias of the gpr register class, so
we have to define instructions for spilling and reloading.

Reviewers: atanasyan

Differential Revision: https://reviews.llvm.org/D38038

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llvm-svn: 318183