Pattern-matched variables in post-inc-icmpzero.ll

Test was failing for clang-native-arm-cortex-a9 build-bot configuration.
The reason for the failure was the test was using hardcoded names.
The attached patch fixes this failure by replacing the hard-coded variables
names with pattern-matched variable names.

Patch by Manish Verma, ARM

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172534 91177308-0d34-0410-b5e6-96231b3b80d8