Fix the logic which determines if the destination Q variable can represent the source Q variable's value with full accuracy.

The new logic is mostly self explanatory except for the value fit checks.

If b has fewer integer bits than a, 0 == (Q_GIABSVAL(a) & (~Q_TC(a, 0) << Q_NIBITS(b))) is checking that a's integer value does not have high-order bits set above what b is capable of storing.

If b has fewer fractional bits than a, 0 == (Q_GFABSVAL(a) & ~(~Q_TC(a, 0) << (Q_NFBITS(a) - Q_NFBITS(b))))) is checking that a's fractional value does not have low-order bits set below what b is capable of storing.

Obtained from: Netflix, Inc.
MFC afer: 1 week