Do not assume the value passed to memset is an i32.

The code in SelectionDAG::getMemset for some reason assumes the value passed to
memset is an i32. This breaks the generated code for targets that only have
registers smaller than 32 bits because the value might get split into multiple
registers by the calling convention. See the test for the MSP430 target included
in the patch for an example.

This patch ensures that nothing is assumed about the type of the value. Instead,
the type is taken from the selected overload of the llvm.memset intrinsic.