JitArm64_LoadStore: Optimize zero stores in stX

The value being stored must be loaded into a register. In the case of an
immediate value, this means it must be materialized. The value is
eventually byteswapped before performing the store.

This can be simplified for the value 0 for two reasons:
- ARM64 has a dedicated zero register, so does not need to be
  materialized.
- Byteswapping zero is still zero, so we can skip this step.

We could skip byteswapping for other values by immediately materializing
the byteswapped value in a register, but the benefits are not so clear
there (if the value needs to be materialized anyway, it is better to do
it up front).

Before:
0x5280001b   mov    w27, #0x0                 ; =0
0xb9404fba   ldr    w26, [x29, #0x4c]
0x12881862   mov    w2, #-0x40c4              ; =-16580
0x0b020342   add    w2, w26, w2
0x5ac00b61   rev    w1, w27
0xb8226b81   str    w1, [x28, x2]

After:
0xb9404fbb   ldr    w27, [x29, #0x4c]
0x12881862   mov    w2, #-0x40c4              ; =-16580
0x0b020362   add    w2, w27, w2
0xb8226b9f   str    wzr, [x28, x2]

0 files changed, 0 insertions, 0 deletions