invalid-LDR_PRE-arm.txt was already passing, but for the wrong reasons. We were...

invalid-LDR_PRE-arm.txt was already passing, but for the wrong reasons.  We were failing to specify enough fixed bits of LDR_PRE/LDRB_PRE, resulting in decoding conflicts.  Separate them into immediate vs. register versions, allowing us to specify the necessary fixed bits.  This in turn results in the test being decoded properly, and being rejected as UNPREDICTABLE rather than a hard failure.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@138653 91177308-0d34-0410-b5e6-96231b3b80d8