convert a report_fatal_error that I was able to trigger into a nice error

so the user at least knows what inline asm is a problem.  For example:

error: inline asm not supported yet: don't know how to handle tied indirect register inputs
pr8788-1.c:14:10: note: generated from here
    asm ("\n" : "+r" (stack->regs)
         ^

Instead of:
fatal error: error in backend: Don't know how to handle tied indirect register inputs yet!

llvm-svn: 100731