Chris convinced me that the default size of the SmallVector (2) was too

Chris convinced me that the default size of the SmallVector (2) was too
small.  Since it doesn't cost much to have 2 more (8 bytes), but not having
them would require a malloc as soon as the third one is needed. Setting
the default to 4 delays the malloc until the 5th parameter attribute.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@35793 91177308-0d34-0410-b5e6-96231b3b80d8