[TableGen][MC] Fix a few places where we didn't hide the underlying type of LaneBitmask very well.

One place compared with 32, which I've replaced with LaneBitmask::BitWidth.

The other places are shifts of a constant 1 by a lane number. But if LaneBitmask were to be a larger type than 32-bits like 64-bits, the 1 would need to be 1ULL to do a 64-bit shift. To hide this I've added a LanebitMask::getLane that hides the shift and make sures the 1 is casted to correct type first.

llvm-svn: 308042