Make getTypeSizeInBits work correctly for array types; it should return

the number of value bits, not the number of bits of allocation for in-memory
storage.

Make getTypeStoreSize and getTypeAllocSize work consistently for arrays and
vectors.

Fix several places in CodeGen which compute offsets into in-memory vectors
to use TargetData information.

This fixes PR1784.

llvm-svn: 97064