[Mips] Make mips32/mips32r2/mips64/mips64r2 real aliases for appropriate
march options.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192289 91177308-0d34-0410-b5e6-96231b3b80d8
[Mips] Do not check for options from the OPT_mips_CPUs_Group. All these
options are aliases now.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192290 91177308-0d34-0410-b5e6-96231b3b80d8
[Mips] Remove unused mips_CPUs_Group options group. It's better to join
all MIPS-related options to the new m_mips_Features_Group later.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192291 91177308-0d34-0410-b5e6-96231b3b80d8
[Mips] Make the isMipsR2Arch() function simpler - remove checking for
MIPS-arch.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192292 91177308-0d34-0410-b5e6-96231b3b80d8
[Mips] Group MIPS-related options checking function in one place.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192293 91177308-0d34-0410-b5e6-96231b3b80d8