[ARM] Better error message for invalid flag-preserving Thumb1 insts

When we see a non flag-setting instruction for which only the flag-setting
version is available in Thumb1, we should give a better error message than
"invalid instruction".

Differential Revision: https://reviews.llvm.org/D27414



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@288805 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed