[MachineCSE] Allow CSE for instructions with ignorable operands

Ignorable operands don't impact instruction's behavior, we can safely do CSE on
the instruction.

It is split from D130919. It has big impact to some AMDGPU test cases.
For example in atomic_optimizations_raw_buffer.ll, when trying to check if the
following instruction can be CSEed

  %37:vgpr_32 = V_MOV_B32_e32 0, implicit $exec

Function isCallerPreservedOrConstPhysReg is called on operand "implicit $exec",
this function is implemented as

  -  return TRI.isCallerPreservedPhysReg(Reg, MF) ||
  +  return TRI.isCallerPreservedPhysReg(Reg, MF) || TII.isIgnorableUse(MO) ||
            (MRI.reservedRegsFrozen() && MRI.isConstantPhysReg(Reg));

Both TRI.isCallerPreservedPhysReg and MRI.isConstantPhysReg return false on this
operand, so isCallerPreservedOrConstPhysReg is also false, it causes LLVM failed
to CSE this instruction.

With this patch TII.isIgnorableUse returns true for the operand $exec, so
isCallerPreservedOrConstPhysReg also returns true, it causes this instruction to
be CSEed with previous instruction

  %14:vgpr_32 = V_MOV_B32_e32 0, implicit $exec

So I got different result from here. AMDGPU's implementation of isIgnorableUse
is

  bool SIInstrInfo::isIgnorableUse(const MachineOperand &MO) const {
    // Any implicit use of exec by VALU is not a real register read.
    return MO.getReg() == AMDGPU::EXEC && MO.isImplicit() &&
           isVALU(*MO.getParent()) && !resultDependsOnExec(*MO.getParent());
  }

Since the operand $exec is not a real register read, my understanding is it's
reasonable to do CSE on such instructions.

Because more instructions are CSEed, so I get less instructions generated for
these tests.

Differential Revision: https://reviews.llvm.org/D137222