nv50/ir/nir: implement nir_alu_instr handling

nv50/ir/nir: implement nir_alu_instr handling

v2: user bitfield_insert instead of bfi
    rework switch helper macros
    remove some lowering code (LoweringHelper is now used for this)
v3: add pack_half_2x16_split
    add unpack_half_2x16_split_x/y
v5: replace first argument with nullptr in loadImm calls
    prefer getSSA over getScratch
v8: fix setting precise modifier for first instruction inside a block
    add guard in case no instruction gets inserted into an empty block
    don't require C++11 features
v9: use CC_NE for integer compares
    convert to C++ style comments
    fix b2f for doubles
    remove macros around nir ops to make it easier to grep them
    add handling for fpow

Signed-off-by: Karol Herbst <kherbst@redhat.com>