AMDGPU/GlobalISel: RBLegalize rules for load

Add IDs for bit width that cover multiple LLTs: B32 B64 etc.
"Predicate" wrapper class for bool predicate functions used to
write pretty rules. Predicates can be combined using &&, || and !.
Lowering for splitting and widening loads.
Write rules for loads to not change existing mir tests from old
regbankselect.

Author: petar-avramovic