[RewriteStatepointsForGC] Strengthen a confusingly weak assertion [NFC]
[RewriteStatepointsForGC] Strengthen a confusingly weak assertion [NFC]

The assertion was weaker than it should be and gave the impression we're growing the number of base defining values being considered during the fixed point interation.  That's not true.  The tighter form of the assert is useful documentation.



git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@247221 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm