Thumb1 does not support dynamic stack realignment.

rdar://10288916 is tracking this fix.

In the past, instcombine and other passes were promoting alloca alignment past
the natural alignment, resulting in dynamic stack realignment.  Lang's work now
prevents this from happening (LLVM commit r141599).  Now that this really 
shouldn't happen report a fatal error rather than silently generate bad code.

llvm-svn: 142028