Cleanup PatternMatch. NFC.

Tidy up the code a little by using 'auto' when the type is obvious, doxify the
comments, and clang-format the file.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223807 91177308-0d34-0410-b5e6-96231b3b80d8
Reorder the code to avoid inserting at the beginning of a vector.

As per dblaikie suggestion, thanks\!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223808 91177308-0d34-0410-b5e6-96231b3b80d8
Remove unneeded curly braces.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223809 91177308-0d34-0410-b5e6-96231b3b80d8
Fix another GCC build failure from r223802

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223810 91177308-0d34-0410-b5e6-96231b3b80d8