nir: introduce lowering of bitfield_insert to bfm and a new opcode bitfield_select.

bitfield_select is defined as:
bitfield_select(mask, base, insert) = (mask & base) | (~mask & insert)
matching the behavior of AMD's BFI instruction.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>

src/compiler/nir/nir.h

@@ -2288,6 +2288,8 @@ typedef struct nir_shader_compiler_options {
    bool lower_bitfield_insert;
    /** Lowers bitfield_insert to compares, and shifts. */
    bool lower_bitfield_insert_to_shifts;
+   /** Lowers bitfield_insert to bfm/bitfield_select. */
+   bool lower_bitfield_insert_to_bitfield_select;
    /** Lowers bitfield_reverse to shifts. */
    bool lower_bitfield_reverse;
    /** Lowers bit_count to shifts. */