[SDAG] convert (sub x, 1) to (add x, -1) in ctpop expansion; NFC
[SDAG] convert (sub x, 1) to (add x, -1) in ctpop expansion; NFC

We canonicalize to the add form, so create that directly for efficiency.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@366914 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm