Set hasSideEffects=0 for PHI and fix affected passes
Previously, hasSideEffects was ? for TargetOpcode::PHI and would be inferred
as 1. D37065 sets the previously inferred properties explicitly. This patch sets
hasSideEffects=0 for PHI, as it is for G_PHI. MachineInstr::isSafeToMove has
been updated so it still returns false for PHI.
Additionally, HexagonBitSimplify relied on a PHI node having the
hasUnmodeledSideEffects property. This patch fixes that assumption.
Differential Revision: https://reviews.llvm.org/D37097