ARM: Diagnostics for out of range fixups.

ARM: Diagnostics for out of range fixups.

Replace some assert() calls w/ actual diagnostics. In a perfect world,
there'd be range checks on these values long before things ever reached
this code. For now, though, issuing a better-late-than-never diagnostic
is still a big improvement over assert().

rdar://11347287

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@155851 91177308-0d34-0410-b5e6-96231b3b80d8