Reduce memory consumption of the JIT by eliminating padding etc

The "struct GenTree_" family is the most obvious example of this, though other important JIT data structures should be checked as well - make sure there is no unnecessary padding (due to data member alignment), see if data members could be rearranged to reduce overall size, and specifically in the case of GenTree_ make sure the sizes of the "small" vs. "large" nodes (and which GenTree* flavors are marked as always having to be "large") are optimized.

category:throughput theme:jit-coding-style skill-level:intermediate cost:small impact:small