[RewriteStatepointsForGC] Adjust naming scheme to be more stable

[RewriteStatepointsForGC] Adjust naming scheme to be more stable

The names for instructions inserted were previous dependent on iteration order.  By deriving the names from the original instructions, we can avoid instability in tests without resorting to ordered traversals.  It also makes the IR mildly easier to read at large scale.

llvm-svn: 243140