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.

llvm-svn: 280011