glsl: Add constant evaluation of bit built-ins.

glsl: Add constant evaluation of bit built-ins.

v2: Order bits from LSB end (31 - count) for ir_unop_find_msb.
v3: Add ir_triop_bitfield_extract as an exception to the op[0]->type ==
    op[1]->type assertion in ir_constant_expression.cpp.

Reviewed-by: Chris Forbes <chrisf@ijw.co.nz> [v2]