R600/SI: Use RegisterOperands to specify which operands can accept immediates

There are some operands which can take either immediates or registers
and we were previously using different register class to distinguish
between operands that could take immediates and those that could not.

This patch switches to using RegisterOperands which should simplify the
backend by reducing the number of register classes and also make it
easier to implement the assembler.

llvm-svn: 225662