Try to avoid creating VGBMs by reusing the permutation mask if it contains a
zero. If the first byte was into (any byte of) a zero vector, then the first
byte of the mask can become zero and reused by putting the mask also as the
first operand. If there instead was a first-byte use of the other source
operand, then that zero index can be reused if the mask is placed as the
second operand.
Review: Ulrich Weigand
Differential Revision: https://reviews.llvm.org/D79925
It's better to reuse the first source value than to use an undef second
operand, because that will make more resulting VPERMs have identical operands
and therefore MachineCSE more successful.
Review: Ulrich Weigand
Jonas Paulsson