R600/SI: Make more store operations legal

v2i32, i32, trunc i32 to i16, and truc i32 to i8 stores are legal for
all address spaces.  We had marked them as custom in order to lower
them for the private address space, but this is no longer necessary.

This enables lowering of misaligned stores of these types in the
DAGLegalizer.

llvm-svn: 228189