[AVR] Do not emit 'LPM Rd, Z' on devices without FeatureLPMX

The 'LPM' instruction has three forms:

------------------------
| form       | feature |
| ---------- | --------|
| LPM        | hasLPM  |
| LPM Rd, Z  | hasLPMX |
| LPM Rd, Z+ | hasLPMX |
------------------------

The second form is always selected in ISelDAGToDAG, even on devices
without FeatureLPMX. This patch emits "LPM + MOV" on devices with
only FeatureLPM.

Reviewed By: jacquesguan

Differential Revision: https://reviews.llvm.org/D141246