[CodeGenPrepare][AArch64] Fix a TLI legality check on iPTR to use a lowered instead.

Fixes PR21548.  Related to PR20474.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221820 91177308-0d34-0410-b5e6-96231b3b80d8