Simplify the PatternMatch unittest by giving it a module, function, and

Simplify the PatternMatch unittest by giving it a module, function, and
basic block to hold instructions, and managing all of their lifetimes in
a fixture. This makes it easy to sink the expectations into the test
cases themselves which also makes things a bit more explicit and clearer
IMO.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198532 91177308-0d34-0410-b5e6-96231b3b80d8