Now that we have operand info for machine instructions, use it to create

temporary registers for things that define a register.  This allows dag->dag
isel to compile this:

int %test() { ret int 65535 }

into:

_test:
        lis r2, 0
        ori r2, r2, 65535
        blr

Next up, getting CopyFromReg to work, allowing arguments and cross-bb values.

llvm-svn: 22932