AMDGPU: Add G_AMDGPU_MAD_64_32 instructions

These generic instructions are trivially selected to
V_MAD_[IU]64_[IU]32 instructions when run on the VALU.

When at least both factors are scalar, it is usually better to execute
some or all of the instruction on the SALU. To this end, we lower the
instruction to simpler instructions that are supported on the SALU
when applying the register bank mapping.

Differential Revision: https://reviews.llvm.org/D124843