Doxygenify some comments.

Clean up cpReg2MemMI and cpMem2RegMI, and doxygenify comments.
Get rid of their uses of SETSW, which is a pseudoinstruction. We can't
JIT-compile pseudoinstructions at the moment. This was blowing up 252.eon/jit,
which has some HUGE stack frames.
Reduce the uses of constantFitsInImmedField().
Consolidate some assertions.

llvm-svn: 15899