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

llvm-svn: 317721