Fix <rdar://problem/8282498> even if it doesn't reproduce on trunk.

When a register is defined by a partial load:

  %reg1234:sub_32 = MOV32mr <fi#-1>; GR64:%reg1234

That load cannot be folded into an instruction using the full 64-bit register.
It would become a 64-bit load.

This is related to the recent change to have isLoadFromStackSlot return false on
a sub-register load.

llvm-svn: 110874