R600/SI: Use S_ADD_U32 and S_SUB_U32 for low half of 64-bit operations

https://bugs.freedesktop.org/show_bug.cgi?id=83416
R600/SI: Fix bug in SIInstrInfo::legalizeOpWithMove()

We must constrain the destination register class of legalized operands
to a VGPR class or else the illegal operand may be folded back into
the instruction by the register coalescer.

This fixes a bug in add.ll that will be uncovered by future commits.