Macro-assembler on aarch64 can merge adjacent loads or stores
into ldp/stp. For example, it can merge:
```
str w20, [sp, #16]
str w10, [sp, #20]
```
into
```
stp w20, w10, [sp, #16]
```

But C2 may generate a sequence like:
```
str x21, [sp, #8]
str w20, [sp, #16]
str x19, [sp, #24]
str w10, [sp, #20]
str x11, [sp, #40]
str w13, [sp, #48]
str x16, [sp, #56]
```
We can't do any merging for non-adjacent loads or stores.