[AMDGPU] Legalize VGPR Rsrc operands for MUBUF instructions

Emit a waterfall loop in the general case for a potentially-divergent Rsrc
operand. When practical, avoid this by using Addr64 instructions.

Recommits r341413 with changes to update the MachineDominatorTree when present.

Differential Revision: https://reviews.llvm.org/D51742

llvm-svn: 343992