Bit Flag Conditional

This MR introduces a bit flag conditional that acts like a ternary operator for a single bit flag. It takes at least three arguments:

• flag_bit specifies which bit of the mask is examined
• mask_expression is an integer-typed expression which acts as a bit mask
• then_expression is a SymPy expression of arbitrary type, and
• optionally else_expression is a SymPy expression of arbitrary type

flag_cond examines the given bit of the given mask and takes the value of then_expression if the bit is set to 1. If not, it becomes either 0 or else_expression:

Three argument version:

    flag_cond(flag_bit, mask, expr) = expr if (flag_bit is set in mask) else 0

Four argument version:

    flag_cond(flag_bit, mask, expr_then, expr_else) = expr_then if (flag_bit is set in mask) else expr_else