ARM: mark UBFX as not allowing PC.

Strictly, it's unpredictable. But we don't quite model that yet and an error is
better than ignoring the issue. This one somehow got left out before though.

rdar://problem/15997748

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211490 91177308-0d34-0410-b5e6-96231b3b80d8