[Hexagon] Make sure that offset on globals matches alignment requirements

A correctly aligned address may happen to be separated into a variable
part and a constant part, where the constant part does not match the
alignment needed in a load/store that uses this address. Such a constant
cannot be used as an immediate offset in an indexed instruction.

When lowering a global address, make sure that if there is an offset
folded into the global, the offset is valid for all uses in load/store
instructions.

llvm-svn: 323562