[mips] Replace usages of register classes with register operands. Also, remove
unnecessary jalr InstAliases in Mips64InstrInfo.td and add the code to print
jalr InstAliases in MipsInstPrinter::printAlias.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187821 91177308-0d34-0410-b5e6-96231b3b80d8