[AArch64] Refactor identification of SIMD immediates

Get rid of icky goto loops and make the code easier to maintain (NFC).

Differential revision: https://reviews.llvm.org/D42723

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@324903 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed