[MemorySSA] Use fewer magic numbers. NFC

INVALID_MEMORYACCESS_ID == 0.

This patch also makes this initialization consistent with the rest of
the "invalid" ones in this file.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@325935 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed