[RS4GC] Re-purpose `normalizeForInvokeSafepoint`; NFC.

`normalizeForInvokeSafepoint` in RewriteStatepointsForGC.cpp, as it is
written today, deals with `gc.relocate` and `gc.result` uses of a
statepoint equally well.  This change documents this fact and adds a
test case.

There is no functional change here -- only documentation of existing
functionality.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@250784 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed