[FastISel][AArch64] Don't fold instructions too aggressively into the memory operation.

Currently instructions are folded very aggressively into the memory operation,
which can lead to the use of killed operands:
  %vreg1<def> = ADDXri %vreg0<kill>, 2
  %vreg2<def> = LDRBBui %vreg0, 2
  ... = ... %vreg1 ...

This usually happens when the result is also used by another non-memory
instruction in the same basic block, or any instruction in another basic block.

If the computed address is used by only memory operations in the same basic
block, then it is safe to fold them. This is because all memory operations will
fold the address computation and the original computation will never be emitted.

This fixes rdar://problem/18142857.