Update update_test_checks to work properly with phi nodes and other fun things.

Summary:
Prior to this change, phi nodes were never considered defs, and so we ended up with undefined variables for any loop.  Now, instead of trying to find just defs, we iterate over each actual IR value in the line, and replace them one by one with either a definition or a use.

We also don't try to match anything in the comment portions of the line.

I've tested it even on things like function pointer calls, etc, and against existing test cases uses update_test_checks
With this change, we are able to use update_tests on the cyclic cases in newgvn.

The only case i'm aware of that will misfire is if you have a string with which contains a valid token.
However, this is the same as it is now, with a slightly larger set of strings that may misfire.
Prior to this change, a test with the string " %a =" would be replaced.

Reviewers: spatel, chandlerc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D28384

llvm-svn: 291357