llvm-reduce: Add reduction for custom register masks

I have a register allocator failure that only reproduces with IPRA
enabled, and requires the specific regmask if I want to only run the
one relevant pass. The printed custom regmask is enormous and I would
like to reduce it.

This reduces each individual bit in the mask, but it would probably be
better to start at register units and clear all aliasing fields at a
time. This would require stricter verification that all aliasing bits
are set in regmasks (although I would prefer to switch regmasks to use
register units in the first place).