[mips] Fold MipsTargetInfoBase subclasses into MipsTargetInfoBase and rename to MipsTargetInfo. NFC
Summary:
This unifies mips/mipsel and mips64/mips64el into a single class so that we can
later support O32 on mips64/mips64el and N32/N64 on mips/mipsel (when an
appropriate CPU selected).
Reviewers: atanasyan
Subscribers: atanasyan, jfb, cfe-commits, dschuff
Differential Revision: http://reviews.llvm.org/D20678
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@270984
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