AMDGPU: Handle i64->v2i32 loads/stores in PreprocessISelDAG

This fixes a select error when the i64 source was also
bitcasted to v2i32 in the original source.

Instead of awkwardly trying to select the modified source value and
the store, replace before isel begins.

Uses a worklist to avoid possible problems from mutating the DAG,
although it seems to work OK without it.

llvm-svn: 248589