Optimizing: Rename `kRegister` to `kCoreRegister`.

And rename `kRegisterPair` to `kCoreRegisterPair`.

Generated with
    sed '-res/\bkRegister\b/kCoreRegister/' \
        --in-place compiler/optimizing/*
    sed '-res/\bkRegisterPair\b/kCoreRegisterPair/' \
        --in-place compiler/optimizing/*
    sed '-res/CPURegister::kCoreRegister\b/CPURegister::kRegister/' \
        --in-place compiler/optimizing/*
in the `art/` directory. The last `sed` command reverts
changes that reference `kRegister` from vixl.

Test: testrunner.py -b --host-gtest --host --optimizing
Flag: EXEMPT PURE_REFACTOR
Change-Id: I506b3d71d6ae8d9131345dc0827bb3ad7d254859