This sets the AllowTruncation flag on isConstOrConstSplat in isNullOrNullSplat, allowing it to see truncated constant zeroes on architectures such as AArch64, where only a i32.i64 are legal. As a truncation of 0 is always 0, this should always be valid, allowing some extra folding to happen including some of the cases from D103755.
Whilst I'm here also add a call to peekThroughBitcasts as bitcast Zero is still Zero, and removed a related TODO comment from isOneOrOneSplat as the bitcast of 1 isn't always still 1.
/*AllowTruncation*/true ?