[AArch64] Move SeparateConstOffsetFromGEPPass before LSR and enable EnableGEPOpt by default.

GEP's across basic blocks were not getting splitted due to EnableGEPOpt
which was turned off by default. Hence, EarlyCSE missed the opportunity
to eliminate common part of GEP's. This can be achieved by simply
turning GEP pass on.
 - This patch moves SeparateConstOffsetFromGEPPass() just before LSR.
 - It enables EnableGEPOpt by default.

Resolves - https://github.com/llvm/llvm-project/issues/50528

Added an unit test.

Differential Revision: https://reviews.llvm.org/D128582