[FastISel][AArch64] Fold 'AND' instruction during the address computation.

The 'AND' instruction could be used to mask out the lower 32 bits of a register.
If this is done inside an address computation we might be able to fold the
instruction into the memory instruction itself.

and  x1, x1, #0xffffffff   ---> ldrb x0, [x0, w1, uxtw]
ldrb x0, [x0, x1]
[FastISel][AArch64] Try to fold the offset into the add instruction when simplifying a memory address.

Small optimization in 'simplifyAddress'. When the offset cannot be encoded in
the load/store instruction, then we need to materialize the address manually.
The add instruction can encode a wider range of immediates than the load/store
instructions. This change tries to fold the offset into the add instruction
first before materializing the offset in a register.