[CodeGen] Optimize AccelTable

Summary:
The class contained arrays of two structures (DataArray and HashData).
These structures were in 1:1 correspondence, and one of them contained
pointers to the other (and *both* contained a "Name" field). By merging
these two structures into one, we can save a bit of space without
negatively impacting much of anything.

Reviewers: JDevlieghere, aprantl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D43073

llvm-svn: 324724