Fix build failures from r337347, found by clang

* Delete a no-longer-used override, and mark the other
getRegisterTypeForCallingConv() as override.
* SPE only supports i32, not i64, as the internal type, so simply remove
the type check, so that DestReg and Opc are provably always set.

GCC 6.4 did not warn about either of the above.

llvm-svn: 337350