[AArch64] AArch64AsmParser clean up of isImmediate functions. NFC

Regression test neon-diagnostics.s needed changing because it now
produces a more specific diagnostic about the immediate ranges. One
change in the expected error message is not obvious, but there multiple
candidate and it happens to pick the immediate diagnostic.

Differential Revision: https://reviews.llvm.org/D29939

llvm-svn: 295331