[AArch64] Teach getTestBitOperand to look through ANY_EXTENDS

This patch teach getTestBitOperand to look through ANY_EXTENDs when the extended bits aren't used. The test case changed here is based what D60358 did to test16 in tbz-tbnz.ll. So this patch will avoid that regression.

Differential Revision: https://reviews.llvm.org/D60482

llvm-svn: 358108