[AMDGPU][GlobalISel] Legalization of G_ROTL and G_ROTR

Add implementation for the legalization of G_ROTL and G_ROTR machine
instructions. They are very similar to funnel shift instructions, the only
difference is funnel shifts have 3 operands, whereas rotate instructions have
two operands, the first being the register that is being rotated and the second
being the number of shifts. The legalization of G_ROTL/G_ROTR is just lowering
them into funnel shift instructions if they are legal.

Patch by: Mateja Marjanovic

Differential Revision: https://reviews.llvm.org/D105347