[TableGen] CodeGenDAGPatterns::GenerateVariants - basic caching of matching predicates

CodeGenDAGPatterns::GenerateVariants is a costly function in many tblgen commands (33.87% of the total runtime of x86 -gen-dag-isel), and due to the O(N^2) nature of the function, there are a high number of repeated comparisons of the pattern's vector<Predicate>.

This initial patch at least avoids repeating these comparisons for every Variant in a pattern. I began investigating caching all the matches before entering the loop but hit issues with how best to store the data and how to update the cache as patterns were added.

Saves around 15secs in debug builds of x86 -gen-dag-isel.

Differential Revision: https://reviews.llvm.org/D51035

llvm-svn: 340837