Tweak hash function and compress hash tables.

Make the hash tables as small as possible while ensuring that all
lookups can be done in less than 8 probes.

Cut the aliases hash table in half by only storing a < b pairs - it
is a symmetric relation.

Use larger multipliers on the initial hash function to ensure that it
properly covers the whole table, and to resolve some clustering in the
very regular ARM register bank.

This reduces the size of most of these tables by 4x - 8x. For instance,
the ARM tables shrink from 48 KB to 8 KB.

llvm-svn: 132888