emit a specific error when the input file is empty. This fixes

emit a specific error when the input file is empty.  This fixes
an annoyance of mine when working on tests: if the input .ll file
is broken, opt outputs an error and generates an empty file.  FileCheck
then emits its "ooh I couldn't find the first CHECK line, scanning
from ..." which obfuscates the actual problem.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@125193 91177308-0d34-0410-b5e6-96231b3b80d8