Correctly handle multi-lined RUN lines.

`utils/update_{llc_test,test}_checks` ought to be able to handle RUN commands
that span multiple lines, as shown in the example at
http://llvm.org/docs/CommandGuide/FileCheck.html#the-filecheck-check-prefix-option

Differential Revision: https://reviews.llvm.org/D26523

llvm-svn: 290716