[TableGen] Avoid repeated find calls in CodeGenDAGPatterns getters. NFCI.
[TableGen] Avoid repeated find calls in CodeGenDAGPatterns getters. NFCI.

The assertion tests were using count() instead of testing the find result, resulting in double the number of searches in debug/assert builds.

Instead, call find once (like the release builds do) and assert the result against end().

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@315151 91177308-0d34-0410-b5e6-96231b3b80d8

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