[FastISel][AArch64] Try to fold the offset into the add instruction when...

[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.

llvm-svn: 218031