[AArch64] Remove an unnecessary restriction on pre-index instructions.

[AArch64] Remove an unnecessary restriction on pre-index instructions.

Previously, the index was constrained to the size of the memory operation for
no apparent reason.  This change removes that constraint so that we can form
pre-index instructions with any valid offset.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@248931 91177308-0d34-0410-b5e6-96231b3b80d8