[Hexagon] Prefer _io over _rr for 64-bit store with constant offset

Identify patterns where the address is aligned to an 8-byte boundary,
but both the base address and the constant offset are both proper
multiples of 4. In such cases, extract Base+4 into a separate instruc-
tion, and use S2_storerd_io, instead of using S4_storerd_rr.

llvm-svn: 277497