[AVR] Fix incorrect expansion of the pseudo 'ELPMBRdZ' instruction

The 'ELPM' instruction has three forms:

--------------------------
| form        | feature  |
| ----------- | -------- |
| ELPM        | hasELPM  |
| ELPM Rd, Z  | hasELPMX |
| ELPM Rd, Z+ | hasELPMX |
--------------------------

The second form is always used in the expansion of the pseudo
instruction 'ELPMBRdZ'. But for devices without ELPMX but only
with ELPM, only the first form can be emitted.

Reviewed By: jacquesguan

Differential Revision: https://reviews.llvm.org/D141221