GlobalISel: use multi-dimensional arrays for legalize actions.

GlobalISel: use multi-dimensional arrays for legalize actions.

Instead of putting all possible requests into a single table, we can perform
the extremely dense lookup based on opcode and type-index in constant time
using multi-dimensional array-like things.

This roughly halves the time spent doing legalization, which was dominated by
queries against the Actions table.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@280011 91177308-0d34-0410-b5e6-96231b3b80d8