[AArch64] Share search bookkeeping in combines. NFCI.
[AArch64] Share search bookkeeping in combines. NFCI.

Share predecessor search bookkeeping in both perform PostLD1Combine
and performNEONPostLDSTCombine. This should be approximately a 4x and
2x performance improvement.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@342986 91177308-0d34-0410-b5e6-96231b3b80d8

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